Radar system and method

ABSTRACT

A radar system for discriminating between sources of radar interference and targets of interest. The system includes a transmitter for transmitting radar signals into a region, a receiver for receiving return signals of the radar signals returned from within the region, and a processor for processing the return signals to discriminate between return signals returned from a first object and return signals returned from a second object where the return signals from the second object comprise both zero and non-zero Doppler components and interfere with the return signals from the first object. The radar system is operable for discriminating between the return signals when the return signals are received at a distance from the second object which is less than a proximity limit based on the geometry of the object.

The invention relates to a radar system and to a method of enhancingradar system capability. The invention relates in particular to a radarsystem with enhanced detection capabilities in a region affected byclutter, structures and moving structures (for example wind turbines)which interfere with radar signals.

There is increasing concern over the effects of new structures, and inparticular large man made structures such as wind turbines, on thecapability of new and existing radar systems, for example air trafficcontrol, marine, and/or air defence systems.

Radar systems are generally designed to differentiate between radarreturns containing reflections from many objects, both moving andstationary. Such reflected signals (collectively termed clutter) may forexample originate from stationary objects such as trees, the ground andeven the wind turbine towers themselves. Whilst existing radars may bedesigned to differentiate between clutter and moving objects based onthe Doppler effect, there are many effects associated with structuressuch as wind turbines which contribute to a significant reduction inradar performance. The fact that large numbers of such structures aretypically arranged in relatively close proximity to one anotherexacerbates the problem.

Wind farms, for example, typically comprise an array of large windturbines, spaced out over an off-shore or inland area that may extendmany kilometres. Each wind turbine typically comprises three principalelements: a tower, a nacelle and a blade assembly. The size andconfiguration of turbines may differ significantly from location tolocation (there are currently in the region of 40 or so differentturbine designs in the UK alone). Generally, however, each turbinecomprises a vertically-mounted blade assembly (having a horizontalrotational axis), and a tower exceeding a height of many tens of metres,or potentially over a hundred metres. The size of such structures,combined with the presence of large moving parts (e.g. the bladeassemblies), means that the turbines act as effective scatterers ofradio signals, with metal towers and/or blade assemblies in particularreflecting a high proportion of the transmitted signal back towards theradar and distorting returns from objects of interest. Thus, theturbines provide spurious moving targets for a radar system and causeshadowing or apparent modulation of signals associated with targets ofinterest, such as aircraft, marine vessels or the like.

Discriminating against spurious moving targets such as those associatedwith a moving blade assembly is complex and as such consumes significantadditional processor time compared, for example, with simple staticclutter reduction or the like.

The large size of the unwanted targets may cause undesirable effectssuch as saturation of a radar receiver, or the like. A large reflection,for example, can result in amplitude limiting within the receiver/signalprocessing thereby causing distortion and possibly resulting in reducedsensitivity and hence degraded detection capability.

Objects located behind the turbine(s) (from the perspective of theradar) may lie in the ‘shadow’ of the turbine. A large portion of theradar energy is blocked by the turbine and is thus lost by reflection inother directions. The radar energy that partially fills the shadowregion behind the turbine (for example by diffraction) thereforerepresents only part of the original signal energy and so the fieldstrength behind the turbine is diminished over a region behind theturbine. Shadowing may therefore result in missed detections.

The rotation of the blades also causes modulation effects, for exampletime modulation of the return signal as the blades present varyingaspect angles, modulation or “chopping” of the radar cross section ofobjects behind the blade (as the blades intermittently obscure thereturns from other objects), and Doppler modulation effects as a resultof the blades' movement in the direction of the radar. Such modulationeffects may cause a wanted target to be missed or to be miss-classified.

Other potential effects include the reflection and re-reflection(cascading reflection) of signals between turbines before they arereturned to the radar.

Degradation in the capability of radar systems such as air trafficcontrol systems to accurately detect and track targets of interestwhilst discriminating against spurious targets is of particular concernbecause of the potential impact on aircraft safety.

There is therefore a need for improvements to enhance the function ofimportant radar systems such as those used in air traffic control andair defence. There is also a more general need for radar systems whichare resistant to the negative effects of large structures and inparticular large man-made structures having moving parts, such as windturbines.

The present invention aims to provide an improved radar system, usefulin this and/or in other cluttered scenarios.

International Patent Application having publication number WO01/059473,which names Cambridge Consultants Ltd as patent applicant and whosedisclosure is incorporated by reference, discloses a radar system whichcomprises apparatus for obtaining positional information relating to anobject, the apparatus comprising: a warning zone definition stage fordefining a warning zone (in two or three dimensions) within a detectionfield of the apparatus; and a discrimination stage for determiningwhether a detected object is within the warning zone; in which thewarning zone is preferably defined as a three-dimensional region withinthe detection field.

International Patent Application having publication number WO97/14058,which names Cambridge Consultants Ltd as patent applicant and whosedisclosure is incorporated by reference, discloses apparatus for andmethod of determining positional information for an object, including amethod for determining the position of an object by means of detectingthe relative timing of probe signals returned by said object at aplurality of spaced apart locations.

Radar Systems for Cluttered Environments

In one aspect of the present invention there is provided a radar systemfor location within a cluttered environment, the radar systemcomprising: means for transmitting (preferably a transmitter) radarsignals into a region (or a volume of interest); means for receiving(preferably a receiver) return signals of said radar signals whenreflected from within said region (or volume of interest), wherein saidtransmitting and receiving means are configured for location within thecluttered environment; and means for processing (preferably a processor)the return signals to extract data for said region including dataassociated with clutter in said region.

The radar system preferably comprises a radar capable of discriminatingtargets in a high clutter environment, for example where the clutter ismore significant or gives greater returns than likely targets ofinterest, and/or where the return signals from the clutter wouldotherwise obscure return signals from targets of interest.

The cluttered environment may include one, some or all of the following:an individual wind turbine (whether off- or on-shore), a wind farm, acollection of wind farms, a ship or groups of ships, sea clutter,buildings and other similar major structures, especially ports, docks,marinas or harbours or the like.

Targets of interest may include aircraft, unmanned aircraft, missiles,road and off-road vehicles, people, pedestrians, boats, ships,submarines.

Targets of interest may also include weather features such as rain,snow, wind and air turbulence.

In another aspect of the present invention a radar system is providedwhich comprises: means for transmitting (preferably a transmitter) radarsignals into a region (or a volume of interest); means for receiving(preferably a receiver) return signals of said radar signals whenreflected from within said region (or volume of interest), wherein saidtransmitting and receiving means are adapted for location on a structureat a wind farm; and means for processing (preferably a processor) thereturn signals to extract wind farm associated data for said region.

It has been appreciated pursuant to the present invention location ofradar sensors at wind farms, whilst counter-intuitive because of thewell known deleterious effects of large metallic structures (andespecially those having moving parts) such as wind turbines on radarsignal processing, has a number of distinct and surprising advantages.It makes use of existing infrastructure (power and mechanical support);it reduces the range of targets within the area of the turbine array;and it increases angular diversity between turbines. In addition,locating a receiver and/or a transmitter at a wind-farm allowsinformation of relevance to the wind farm itself to be extracted. Forexample, this may include information on objects (such as marine vesselsor aircraft) moving in a volume of interest in close proximity to thewind-farm, to be detected where local air traffic control, navel, or airdefence radar would have difficulty.

The transmitting means may comprise a static transmitter.

Preferably the transmitting means has a first aperture and the receivingmeans a second aperture such that said first aperture is of a differentsize to said second aperture. The first aperture is preferably smallerthan said second aperture. The receiving means may comprise a pluralityof sub-arrays each of which may have a sub-aperture of substantiallyequal size and shape to the first aperture.

The transmitting means is preferably configured to persistentlyilluminate said region, preferably without being sequentially scanned ordirected.

The processing means may be configured for forming multiple receivingbeams.

Coherent integration of return signals may occur subject to a limit onthe range and/or range rate associated with a corresponding observationrepresented by the return signals. The limit may be inverselyproportional to an operating frequency of the radar system and/or may beproportional to the square of the speed of light. The range may belimited in inverse proportion to the maximum magnitude of the range rateand/or the range rate may be limited in inverse proportion to themaximum range.

The limit is preferably expressed by the inequality:

$\left( {{\frac{R}{t}}\left( \max \right) \times {R\left( \max \right)}} \right) \leq \frac{c^{2}}{\left( {8 \times F_{op}} \right)}$

where R is the range and dR/dt is the range rate associated with theobservation, c is the speed of light, and F_(op) is operating frequencyof the radar system.

The transmitting means is preferably configured to illuminate said wholeregion with a broad beam, may be configured to illuminate a whole volumeof interest simultaneously, may be configured to illuminate said regionwith a coherent signal modulated to permit range resolution, and/or maybe configured to illuminate said region with a coherent signal modulatedas a regular sequence of pulses. The transmitting means may beconfigured to illuminate targets in the region at a rate (for example apulse rate) sufficient to exceed the Nyquist limit for Doppler shiftsassociated with the targets.

The processing means may be configured to assess the significance of anobservation represented by a return signal preferably only after datarelating to the observation has been extracted, stored, and analysed.The processing means may be configured to classify a target representedby an observation only after data relating to the observation has beenextracted, stored, and analysed. The processing means may be configuredto identify observations of interest from said extracted data andpreferably stores historical data for said identified observations.

The historical data may comprise phase and/or amplitude histories. Theprocessing means may be configured to form tracks for said targets basedon processing and interpretation of said historical data and/or may beconfigured to discriminate between significant and insignificantobservations (and/or targets representing one class or another) based onsaid historical data.

The processing means may be configured to store extracted datarepresenting an observation in process pixels each of which represents aunique set of attributes. The attributes for each pixel may comprise acombination comprising at least two of time, range, range rate and/orDoppler frequency for the associated observation. The attributes foreach pixel may comprise at least one of beam number, sub-array numberand/or element number for the associated observation. The processingmeans may be configured to store a characteristic of a return signalrepresenting the observation in an associated pixel. The characteristicmay comprise at least one of amplitude, phase and frequency. Theobservation may represent one of a target, an item of clutter, or a‘null’.

The receiving means preferably comprises at least one array comprising aplurality of receiving elements (or sub-arrays), each element may beconfigured to receive signals from substantially a whole volume ofinterest, thereby forming an associated signal channel. The receivingmeans may comprise a plurality of said arrays (or sub-arrays).

The processing means may be configured for forming a plurality of beamsby combining different signal channels with suitable amplitude and/orphase weightings. The processing means may be configured for forming aplurality of beams having substantially a different look direction. Theprocessing means may be configured for forming a plurality of apertureswith beams having substantially the same look direction. The beams arepreferably formed for each of a plurality receiving elements. The beamsmay be formed for each of the plurality of sub-arrays of receivingelements.

The processing means may be configured for monopulse angular measurementusing a plurality of the beams. The monopulse angular measurement maycomprise phase monopulse angular measurement. The monopulse angularmeasurement may comprise amplitude monopulse angular measurement.

The receiving means may have a substantially larger total aperture thansaid transmitting means. The processing means may be configured fordetermining the amplitude, frequency, delay and/or phase of said returnsignals using a signal which is coherent with the transmitted radarsignal.

The receiving means may comprise a planar array of receiving elementsand/or may comprise a non-planar array of receiving elements conformalto a known shape.

The radar system is preferably located at the wind farm. The radarsystem preferably comprises a holographic radar.

The processing means may be configured to process signals received bythe sub-arrays in a first data stream and a second data stream inparallel. Each data stream may be processed using different amplitudeand/or phase weightings. The amplitude and/or phase weightings used forthe first data stream may be configured to provide a null in a directionof a land or sea surface thereby to reject surface targets. Similarly,the amplitude and/or phase weightings used for the second data streammay be configured to provide a null in the direction of raised objectsthereby to reject such objects in favour of surface targets.

Asymmetric Aperture Aspects

The transmitting means preferably has a first aperture; and saidreceiving means preferably has a second aperture; wherein said secondaperture is preferably of a different size to said first aperture.

According to another aspect of the present invention there is provided aradar system for location in a cluttered environment, the radar systemcomprising: means for transmitting (preferably a transmitter) radarsignals into a region, said transmitting means having a first aperture;means for receiving (preferably a receiver) return signals of said radarsignals, reflected from within said region, said receiving means havinga second aperture; and means for processing (preferably a processor) thereturn signals to extract data including clutter related data; whereinsaid second aperture is of a different size to said first aperture.

The transmitting means may comprise a static transmitter.

Preferably the transmitting means has a first aperture and the receivingmeans a second aperture such that said first aperture is of a differentsize to said second aperture. The first aperture is preferably smallerthan said second aperture. The receiving means may comprise a pluralityof sub-arrays each of which may have a sub-aperture of substantiallyequal size and shape to the first aperture.

The transmitting means is preferably configured to persistentlyilluminate said region, preferably without being sequentially scanned ordirected.

The processing means may be configured for forming multiple receivingbeams.

Coherent integration of return signals may occur subject to a limit onthe range and/or range rate associated with a corresponding observationrepresented by the return signals. The limit may be inverselyproportional to an operating frequency of the radar system and/or may beproportional to the square of the speed of light. The range may belimited in inverse proportion to the maximum magnitude of the range rateand/or the range rate may be limited in inverse proportion to themaximum range.

The limit is preferably expressed by the inequality:

$\left( {{\frac{R}{t}}\left( \max \right) \times {R\left( \max \right)}} \right) \leq \frac{c^{2}}{\left( {8 \times F_{op}} \right)}$

where R is the range and dR/dt is the range rate associated with theobservation, c is the speed of light, and F_(op) is operating frequencyof the radar system.

The transmitting means is preferably configured to illuminate said wholeregion with a broad beam, may be configured to illuminate a whole volumeof interest simultaneously, may be configured to illuminate said regionwith a coherent signal modulated to permit range resolution, and/or maybe configured to illuminate said region with a coherent signal modulatedas a regular sequence of pulses. The transmitting means may beconfigured to illuminate targets in the region at a rate (for example apulse rate) sufficient to exceed the Nyquist limit for Doppler shiftsassociated with the targets.

In-Fill Application Aspects

The radar system is preferably configured for detecting objects in asurveillance area; the region is preferably a region within thesurveillance area, which has a detection capability which is degraded bywind farm associated interference; the return signals may therefore bereflected from objects located within the region; and the processingmeans is preferably configured for extracting wind farm associated datafor the objects and for analysing the wind farm associated data toenhance detection of the objects within the region.

The radar system preferably further comprises primary means forreceiving (preferably a receiver) radar signals reflected from an objectwhen said object is located within a surveillance area; wherein thereceiving means which is adapted for location at the wind farm is asecondary means for receiving (preferably a receiver) return signalsreflected from an object when said object is located within a regionwithin the surveillance area, wherein said region has a detectioncapability which is subject to wind farm associated degradation whencompared to the rest of the surveillance area; and wherein theprocessing means is configured for: (i) processing said return signalsreceived by said primary receiving means to detect said object withinsaid surveillance area; (ii) for processing said signals received bysaid secondary receiving means to extract said wind farm associated datafor said object when said object is located within said region; and(iii) for analysing said wind farm associated data to enhance thedetection capability within said region.

According to another aspect of the present invention there is provided aradar system (or service) adapted to operate in the presence of primarymeans for receiving (preferably a receiver) radar signals reflected froman object of interest within a surveillance area; and comprisingsecondary means for receiving (preferably a receiver) radar signalsreflected from said object when said object is located within aclutter-affected region within said surveillance area; and means forprocessing said signals received by said secondary receiving means todetect said object within said region; wherein said processing means isconfigured to process said signals received by said secondary receivermeans to enhance detection within said region and to provide the resultsto said primary means.

According to another aspect of the present invention there is provided aradar system comprising: primary means for receiving (preferably areceiver) radar signals reflected from an object of interest within asurveillance area; secondary means for receiving (preferably a receiver)radar signals reflected from said object when said object is locatedwithin a region within said surveillance area; and means for processingsaid signals received by said primary receiving means to detect saidobject within said surveillance area; wherein said processing means isconfigured to process said signals received by said secondary receivermeans to enhance detection within said region.

Thus the radar system advantageously augments the function of existingand/or new surveillance radar systems in the presence of new structures,for example to ameliorate the effect of wind farms on air trafficcontrol radar systems. Advantageously, the secondary receiving meansprovides additional coverage to fill in areas degraded by the wind farm(or other such group of interfering structures). Preferably thesecondary receiving means includes a transmitting element arranged suchthat it illuminates the wind farm itself in a way that does not suffersuch degradation.

Preferably the secondary receiving means comprises a suitable form of aradar sensor (or group of sensors) mounted at a wind farm (or the like)for example attached to a turbine (or group of turbines).

A preferred form of the radar sensor is a static sensor (i.e. one thatdoes not require a rotating antenna) thereby avoiding mechanicalinterference with the turbines. A static sensor has the furtheradvantages of ease of installation and reduced susceptibility to theharsh environment to be expected at a wind farm. Many wind farms, forexample, are sited offshore and as such are subjected to particularlysevere weather and stormy seas.

The radar sensor may comprise a static array of transmitting and/orreceiving elements (for example similar to that described inWO01/059473) whose region of sensitivity may be adjusted. Morespecifically the sensitivity of the sensor array may be adjustable todefine a region which coincides with a region of reduced detectioncapability (or degraded radar performance). The ability of the sensorarray to measure the position of targets is preferably provided bycalculation of amplitude and/or phase relationships (and/or) delaysbetween signals received at different elements or combinations ofelements of the receiving array (for example as described inWO97/14058).

Each radar sensor preferably has a wide field of view and can measuredirections in both azimuth and elevation. Wider angular coverage 360degree coverage may be provided by installing two or more radar sensorscomprising, for example, planar antenna arrays pointed appropriately.Alternatively or additionally wider angular coverage may be provided byone or more radar sensors comprising, for example non-planar arrays. Inthe case of a wind farm the radar sensors may be positioned separately,may be located at different positions around the perimeter of a turbinesupport shaft, or may be attached to different turbine supports.

Data related to targets detected by the radar sensors may becommunicated by a wireless link to processing means associated with theprimary transmitter/receiver (e.g. main air traffic control system orsystems) for integration with similar data generated by other radarsensors. The processing means may comprise suitable computer software orthe like.

The integration of target data is preferably simplified for example bydefining a detection zone for the secondary receiver means whichsubstantially matches a region of reduced radar performance. Thedetection zone may be defined in a similar manner to the ‘warning zone’described in WO01/059473.

The region is preferably a region having a reduced detection capability.The detection capability may be degraded by interference from at leastone structure or a multiplicity of such structures. The structures mayhave at least one moving part and/or may have a size comparable to orlarger than the object detected (indeed the size may be significantlylarger than the object detected). The or each structure may be capableof causing multiple multi-path and/or cascading reflections (either inisolation or in combination with other such structures) and/or may be aman made structure. The structure(s) may be largely metallic and/or maybe designed for the production of electricity. The or each structure ispreferably a wind turbine.

The or each secondary receiving means may be located on the or at leastone of the structure(s).

The radar system may comprise means for communicating data correspondingto the radar signals received by the secondary receiving means to theprocessing means. The communicating means may comprise wireless oroptical communicating means.

The secondary receiving means may be located remotely from the primaryreceiving means at a location within or at the edge of the region.

The processing means may comprise a detection zone definition stage fordefining a detection zone for said secondary receiving means within adetection field of said secondary receiving means. The processing meansmay comprise a discrimination stage for determining whether a detectedobject is within the detection zone. The detection zone may be definedas being substantially coincident with said region. The detection zonemay be contained within and may be smaller than the detection field ofthe secondary receiving means. The shape of the detection zone may bedissimilar to the shape of the detection field of the secondaryreceiving means. The shape of the detection zone may be non-circular ornon-spherical. The detection zone definition stage may include analgorithm that defines a detection zone as a function of a coordinatewithin the detection field.

The processing means may comprise an object location stage preferablyfor determining the position of a detected object within the detectionfield of the apparatus. The discrimination stage may include acoordinate generating stage for generating a coordinate of a detectedobject, which coordinate may then compared with the detection zone.

The discrimination stage may be operable to determine the coordinates ofthe detected object and preferably to compare the determined coordinateswith the coordinates of the detection zone preferably to determinewhether the object is within the detection zone.

The detection zone definition stage may define at least a limiting valueof one or more ordinates of a coordinate within the detection field. Thedetection zone definition stage may define at least a limiting value ofone or more angles of a polar coordinate within the detection field. Thedetection zone definition stage may define at least a limiting value ofa range of a polar coordinate within the detection field. The detectionzone may include a plurality of discontinuous spatial regions. Thedetection zone may be limited in range and/or may be approximatelycuboid.

The discrimination stage may be operative to generate an output signalindicative that the object is within the detection zone. Thediscrimination stage may be operable to apply different logic to atleast two of the zones.

The detection zone definition stage may define a plurality ofnon-coextensive detection zones, and preferably in which thediscrimination stage is operative to generate an output signalindicative of which of the plurality of detection zones contains theobject.

The discrimination stage may be operative to analyse a characteristic ofan object outside of the detection zone and/or may be operable to trackan object outside the detection zone and to predict its entry into thedetection zone.

The processing means may be configured for definition and redefinitionof said detection zone in dependence on requirements.

The secondary receiving means preferably comprises an antenna arrayhaving at least one (preferably two) receiving elements for receivingsaid reflected radar signals. The array may comprise at least onetransmitting element for transmitting radar signals for reflection fromsaid object of interest.

The antenna array may be a planar array or may be a non-planar array.The antenna array may be arranged for receiving a plurality of signalsindicative of an azimuth of the object and wherein said processing meansmay be configured for determining said azimuth from said signals.

The antenna array may be arranged for receiving a plurality of signalsindicative of an elevation of said object and wherein said processingmeans may be configured for determining said elevation from saidsignals.

The radar system may comprise a plurality of the secondary receivingmeans arranged in geographical association with said region (preferablyat different locations within and/or at the edge of said region).

According to another aspect of the invention there is provided a radarsystem for enhancing detection of an object within a region of asurveillance area, wherein detection capability within said region isdegraded by interference caused by at least one structure; the radarsystem comprising: means for receiving (preferably a receiver) radarsignals reflected from said object when said object is located withinsaid region; wherein said receiving means is located on the or at leastone of the structure(s).

The detection capability may be degraded by interference from amultiplicity of the structures. The or each structure may have at leastone moving part and/or may have a size comparable to or larger than (orsignificantly larger than) the object detected. The or each structuremay be capable of causing multiple multi-path and/or cascadingreflections (either in isolation or in combination with other suchstructures). The or each structure may be a man made structure and/ormay be a largely metallic structure. The or each structure may bedesigned for the production of electricity. The or each structure ispreferably a wind turbine. The or each secondary receiving means may belocated on the or at least one of the structure(s).

The radar system preferably comprises a plurality of the receivingmeans, each of the receiving means being arranged on the or at least oneof the structure(s).

According to another aspect of the invention there is provided a methodof enhancing radar system capability comprising: receiving radar signalsreflected from an object of interest within a surveillance area at aprimary receiving means; receiving radar signals reflected from saidobject when said object is located within a region within saidsurveillance area at a secondary receiving means; integrating saidsignals received at said primary receiving means with said signalsreceived at said secondary receiving means to enhance detection withinsaid region.

Environmental Application Aspects

The returned signals preferably comprise indicators of prevailingenvironmental conditions in the region; and the processing means ispreferably configured for extracting wind farm associated data for saidindicators and preferably for analysing said data to determine operatingparameters for said wind farm.

According to another aspect of the invention there is provided a radarsystem comprising: means for transmitting (preferably a transmitter)radar signals into a region; means for receiving (preferably a receiver)return signals of said radar signals reflected from within said region,wherein said transmitting and receiving means are adapted for locationon a structure at a wind farm; and means for processing (preferably aprocessor) the return signals to extract wind farm associated data forsaid region; wherein the returned signals comprise indicators ofprevailing environmental conditions in said region, and said processingmeans is configured for extracting wind farm associated data for saidindicators and for analysing said data to determine operating parametersfor said wind farm.

The indicators may comprise indications of changes in air borne moistureand/or precipitation characteristics and/or may comprise indications offluid flow characteristics. The fluid flow characteristics may comprisecharacteristics of air flow, may comprise wind shear and/or turbulencecharacteristics, and/or may comprise characteristics of vertical airflow stratification.

The processing means may be configured for processing said returnsignals to resolve different layers of vertical air flow stratificationusing, for example, vertical receiver beam-forming.

The receiving means may comprise an array of receiving elements andprocessing means may be configured for processing said return signals toresolve different layers of vertical air flow stratification byanalysing Doppler frequencies and/or phases across the receiving array.

The fluid flow characteristics may comprise characteristics of watermovement, for example, characteristics of waves. The processing meansmay be configured for processing the return signals to discriminatebetween indicators of air flow characteristics and indicators of surfacecharacteristics. The surface characteristics may comprisecharacteristics of waves.

The processing means may be configured to output signals for controllingsaid operating parameters. The control signals may comprise signals formodifying the pitch of at least one blade of at least one wind turbine,may comprise signals for modifying the pitch of the at least one bladeover time as the blade rotates, and or may comprise signals formodifying the direction at which at least one wind turbine faces. Thecontrol signals may comprise signals for feathering the blades of atleast one wind turbine, may comprise visual or audio signals foralerting an operator to said operating parameters, and or may comprisesignals for interpretation by a controller for automatic control of saidoperating parameters.

The processing means may be configured for extracting information fromsaid indicators for use in estimating the future power output of aenergy generation facility (for example a wind turbine or wind farm)

Fresnel Zone Clutter De-Emphasis

According to another aspect of the invention there is provided a radarsystem (e.g. radar apparatus) for discriminating between sources ofradar interference (e.g. in a cluttered or highly cluttered environment)and targets of interest, the radar system comprising: means fortransmitting (preferably a transmitter) radar signals into a region;means for receiving (preferably a receiver) return signals of said radarsignals returned from within said region; and means for processing(preferably a processor) the return signals to discriminate betweenreturn signals returned from a first object and return signals returnedfrom a second object preferably where said return signals from saidsecond object comprise both zero and non-zero Doppler components andpreferably where said returns from said second object interfere withsaid return signals from said first object.

As used herein the term “Doppler components” preferably connotes ameasure of a Doppler shift, so that preferably, a zero-Doppler componentdenotes a stationary target and a non-zero Doppler component denotes amoving target. Furthermore, as used herein the term “interference”preferably connotes not merely destructive and constructive interferencebut more widely any circumstance in which one signal masks or otherwiseobscures another signal.

The radar system is preferably operable for discriminating between thereturn signals (from the first and second objects) at a distance fromthe second object which is preferably less than a predefined proximitylimit which may be based on the geometry of the object and/or may bebased on the wavelength (and hence the frequency) of the signaltransmitted by the transmitter means.

According to another aspect of the invention there is provided a radarsystem for discriminating between sources of radar interference andtargets of interest, the system comprising: means for transmitting(preferably a transmitter) radar signals into a region; means forreceiving (preferably a receiver) return signals of said radar signalsreturned from within said region; and means for processing (preferably aprocessor) the return signals to discriminate between return signalsreturned from a first object and return signals returned from a secondobject wherein said return signals from said second object comprise bothzero and non-zero Doppler components and interfere with said returnsignals from said first object; wherein said radar system is operablefor discriminating between said return signals when said return signalsare received at a distance from said second object which is less than aproximity limit based on the geometry of the object.

The radar system may be adapted to discriminate between the returnsignals where the second object has an effective radar cross-sectionwhen observed from a distance greater than the proximity limit which isgreater than an effective radar cross-section of the first object.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably a distance within which aneffective radar cross-section of the second object varies with thedistance of the transmitting and/or receiving means from the secondobject.

The processing means may be operable to carry out the discriminationwhere the return signals from the second object comprise signalsreturned from a distance at which the effective radar cross-section ofthe second object is preferably substantially less than a theoreticalobservable radar cross section when observed from an infinite distance.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably dependent on at least onedimension of the second object substantially perpendicular to a line ofsight of the transmitting means.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably substantially dependent on thesquare of the dimension of the second object. The dimension may be adimension of a moving part of the second object and/or may be adimension of a rotating part of the second object.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably dependent on a wavelength of thesignals transmitted by the transmitting means. The proximity limit maybe inversely proportional to the wavelength of the signals transmittedby the transmitting means.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably given substantially by theequation:

$D_{p} \approx {\frac{2}{\lambda}r_{tg}^{2}}$

where D_(P) is the proximity limit, λ is a (or the) wavelength of thetransmitted signal, and r_(tg) is a (or the) dimension target.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably dependent on the size of a zoneat the second object across which returns from features of the objectexhibit a phase deviation of less than 180°.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably dependent on the size of a zoneat the second object across which returns from the object exhibit adeviation of less than half a wavelength.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably determined based on a comparisonof a size of the zone (a radius) with the geometry (preferably adimension) of the second object (preferably in a plane perpendicular toa line of sight of the transmitting and/or receiving means). The zonepreferably comprises a (e.g. the first) Fresnel zone at said object. Thesecond object may comprise a wind turbine or a part thereof.

The second object may comprise a blade of a wind turbine.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably dependent on the square of alength of the blade divided by the wavelength of the transmitted signal.

The radar system may be adapted to operate within the proximity limitwhere the proximity limit is preferably given substantially by theequation:

$D_{p} \approx {\frac{2}{\lambda}L_{B}^{2}}$

where D_(p) is the proximity limit, λ is the wavelength of thetransmitted signal, and L_(B) is the length of the blade.

The transmitting means may comprise a static transmitter. Thetransmitting means may have a first aperture and the receiving means mayhave a second aperture. The first aperture may be of a different size tothe second aperture (for example, smaller than the second aperture).

The receiving means may comprise a plurality of sub-arrays each of whichmay have a sub-aperture of substantially equal size and shape to thefirst aperture.

The transmitting means may be configured to persistently illuminate theregion (for example, without being sequentially scanned or directed).

The processing means may be configured for forming multiple receivingbeams.

Coherent integration of return signals may occur subject to aholographic limit on the range and/or range rate associated with acorresponding observation represented by the return signals.

The holographic limit may be inversely proportional to an operatingfrequency of the radar system and/or may be proportional to the squareof the speed of light.

Compliance with the holographic limit may require range to be limited ininverse proportion to the maximum magnitude of the range rate and/or mayrequire the magnitude of range rate to be limited in inverse proportionto the maximum range.

The limit may be expressed by the inequality:

$\left( {{\frac{R}{t}}\left( \max \right) \times {R\left( \max \right)}} \right) \leq \frac{c^{2}}{\left( {8 \times F_{op}} \right)}$

where R is the range and dR/dt is the range rate associated with theobservation, c is the speed of light, and F_(op) is operating frequencyof the radar system.

The transmitting means may be configured to illuminate targets in theregion at a rate (for example a pulse rate) sufficient to exceed theNyquist limit for Doppler shifts associated with the targets.

The processing means may be configured to identify observations ofinterest from the extracted data and to store historical data for theidentified observations (for example, phase and/or amplitude histories).

The processing means may be configured to discriminate betweensignificant and insignificant observations (and/or targets representingone class or another) based on the historical data.

The processing means may be configured to store extracted datarepresenting an observation in process pixels each of which mayrepresent a unique set of attributes (for example, a combinationcomprising at least two of time, range, range rate and/or Dopplerfrequency for the associated observation).

The attributes for each pixel may comprise may be at least one of beamnumber, sub-array number and/or element number for the associatedobservation.

The receiving means may comprise at least one array comprising aplurality of receiving elements (or sub-arrays), each element may beconfigured to receive signals from substantially a whole volume ofinterest, thereby to form an associated signal channel.

The processing means may be configured for forming a plurality of beamsby combining different signal channels with suitable amplitude and/orphase weightings. The processing means may be configured for forming aplurality of beams, for example having substantially a different lookdirection.

The processing means may be configured for forming a plurality ofapertures with beams having substantially the same look direction (forexample, substantially parallel beams).

The beams may be formed for each of a plurality receiving elementsand/or may be formed for each of the plurality of sub-arrays ofreceiving elements.

The processing means may be configured for phase and/or may beconfigured for amplitude monopulse angular measurement using a pluralityof the beams.

The receiving means may comprise an array of receiving elements (forexample, a planar array of receiving elements or a non-planar array ofreceiving elements conformal to a known shape).

The radar system may be located at a wind farm. The radar system maycomprise a holographic radar.

According to another aspect of the invention there is provided acombination of a radar system according to any preceding aspect and thesecond object wherein the second object has a given geometry. The givengeometry may, for example, comprise a given turbine blade length.

The return signals from each said object may comprise at least oneDoppler component and the processing means may be operable fordiscriminating between the return signals in dependence on a spread ofthe Doppler components for each object.

Discrimination Based on Spread of Doppler Spectrum

According to another aspect of the invention there is provided a radarsystem for discriminating between sources of radar interference andtargets of interest, the system comprising: means for transmitting(preferably a transmitter) radar signals into a region; means forreceiving (preferably a receiver) return signals of said radar signalsreturned from within said region; and means for processing (preferably aprocessor) the return signals to discriminate between return signalsreturned from a first object and return signals returned from a secondobject wherein said return signals from said objects comprise at leastone Doppler component; wherein said processing means is operable fordiscriminating (or comprises means for discriminating e.g. adiscriminator) between said return signals in dependence on a spread ofsaid Doppler components for each object.

The processing means may be operable to determine that said returnsignals are returned from said second object if said at least oneDoppler component comprises a plurality of components at a plurality oflocations across a pre-defined Doppler spectrum.

The processing means may be operable for discriminating between saidreturn signals in dependence on said spread of Doppler components in asingle observation.

The processing means may be operable to determine that said returnsignals are returned from said first object if said at least one Dopplercomponent comprises a localised part of a (or the) pre-defined Dopplerspectrum.

The processing means may be operable to place said at least one Dopplercomponent into at least one of a plurality of discrete Doppler bins andto discriminate between said return signals in dependence on the or eachDoppler bin in which said at least one Doppler component is preferablylocated.

The processing means may be operable to operable to determine that saidreturn signals are returned from said second object if said at least oneDoppler component of said return signal comprises a plurality of Dopplercomponents located in a number (preferably a proportion) of saidplurality of Doppler bins which is preferably not less then a firstpredetermined threshold (for example, a threshold between 5% and 100% ofthe plurality of Doppler bins).

The processing means may be operable to determine that said returnsignals are returned from said first object if said at least one Dopplercomponent of said return signal is preferably located in a number(preferably a proportion) of Doppler bins which does not exceed a secondpredetermined threshold (for example, a threshold comprising anywherefrom a single Doppler bin to 1%, 2%, 5% or up to 25% of the plurality ofDoppler bins).

The number of Doppler bins may represent a target of interest comprisingan environmental target (for example, rain, snow or wind) (which may bewind farm associated) and said processing means may be configured toextract information relating to said target for use in estimating thefuture power output of an energy generation facility (for example a windturbine or wind farm).

The processing means may be operable to discriminate between said returnsignals in dependence on an evolution of Doppler characteristicsexhibited by said objects over time.

The evolution of Doppler characteristics may be related to the evolutionof the signal in the time domain.

The evolution of the signal in the time domain may take the form offlashes (for example, from a rotating object such as a wind turbineblade or the like)

The processing means may be operable to discriminate between said returnsignals in dependence on the conformity of said evolution of Dopplercharacteristics with a model or function.

The model or function may comprise a sinusoidal, exponential, quadratic,and/or logarithmic model or function.

The system may comprise means for determining a range of said objectsbased on said return signals, and said processing means may be furtheroperable to discriminate between said return signals in dependence on achange in said determined range of the objects over time.

The processing means may be operable to determine that said returnsignals are returned from said first object if said range changes over atime period.

The processing means may be operable to determine that said returnsignals are returned from said second object if said range remainssubstantially constant over a time period.

The transmitter means may be operable to transmit said radar signals inan transmitter beam directed upwardly at no less than 45° relative to ahorizon; said receiving means may be operable to detect return signalsof said radar signals returned from an airborne object within saidupwardly directed transmitter beam; and said processing means may beoperable to process the return signals returned from said airborneobject thereby to detect and track said airborne object.

Vertically Facing Radar

According to another aspect of the invention there is provided a radarsystem for detecting and tracking an airborne object the systemcomprising: means for transmitting (preferably a transmitter) radarsignals in an transmitter beam directed upwardly at no less than 45°relative to a horizon; means for receiving (preferably a receiver)return signals of said radar signals returned from an airborne objectwithin said upwardly directed transmitter beam; and means for processing(preferably a processor) the return signals returned from said airborneobject thereby to detect and track said airborne object.

The upwardly directed transmitter beam preferably comprises asubstantially vertically directed transmitter beam.

The transmitting means may be configured for transmitting further radarsignals in at least one further transmitter beam directed at an angle ofno more than 45° relative to the horizon; said receiving means may beconfigured for receiving return signals of said further radar signalsreturned from an object within the at least one further transmitterbeam; and said processing means may be operable to process said returnsignals received by said receiving means thereby to detect and trackobjects within said upwardly and/or said at least one further beam.

The at least one further transmitter beam may comprise a plurality oftransmitter beams each preferably directed at an angle of no more than45° relative to the horizon and at a different angle in azimuth.

Each further transmitter beam may be directed at substantially a 90°(and/or a 180°) angle in azimuth relative to at least one other furthertransmitter beam.

The transmitter means may be configured such that said furthertransmitter beams are directed to illuminate a volume from substantiallythe same location within the volume.

The transmitter means may be configured such that said furthertransmitter beams are directed to illuminate a volume from differentlocations within the volume or at a perimeter of the volume.

The radar system may be configured to process signals returned from saidairborne object and to discriminate them from signals returned fromclutter objects (for example, sources of radar interference) containingrotating components.

The receiving means may be operable to receive return signals of saidradar signals returned from within said region via an array of receiverelements and in a plurality of channels each corresponding to at leastone of said receiver elements, and the processing means may be operableto process the return signals to form (or represent) concurrently aplurality of beams in the frequency domain, the plurality of beamscomprising at least one beam for each channel.

Time-Frequency Transformation Prior to Beamforming

According to a further aspect of the present invention there is provideda radar system for discriminating between sources of radar interferenceand targets of interest, the system comprising: means for transmitting(preferably a transmitter) radar signals into a region; means forreceiving (preferably a receiver) return signals of said radar signalsreturned from within said region via an array of receiver elements andin a plurality of channels each corresponding to at least one of saidreceiver elements; and means for processing (preferably a processor) thereturn signals to form (or represent) concurrently a plurality of beamsin the frequency domain, the plurality of beams comprising at least onebeam for each channel.

Preferably, the processing means is configured for transforming thereceived return signals from the time domain into the frequency domain(for example, using a Fourier transform such as the so called fastFourier transform/FFT).

Preferably, the processing means is configured for forming the beamsonly after said transformation from the time domain into the frequencydomain is completed.

Preferably, the processing means is configured for detecting any targetsin each beam.

Preferably, the processing means is configured for forming a migrationsurface (for example, a range-range rate surface) for each beam soformed and for detecting targets using the migration surfaces.

Preferably, the processing means is configured for determining anangular measurement for a target detected in at least one of the beams.

Preferably, the angular measurement comprises a phase monopulse angularmeasurement, or amplitude monopulse angular measurement.

Preferably, the processing means is configured for re-forming the beamsprior to the angular measurement.

Preferably, the beam formation and the beam re-formation are based onthe same frequency domain data.

Preferably, the angular measurement is determined using a plurality ofre-formed beams.

Preferably, the transmitting means is operable to transmit radar signalsinto a region from an array of transmitter elements; the receiving meansis operable to receive return signals of the radar signals returned fromwithin the region via an array of receiver elements and in a pluralityof channels each corresponding to at least one of the receiver elements;and the processing means is operable to process the return signals toform a receiver beam for each of said channels; and wherein the numberof transmitter elements in the array of transmitter elements is greaterthan the number of receiver elements to which each channel corresponds.

Broad Beam Transmitter

According to a further aspect of the invention, there is provided aradar system for discriminating between sources of radar interferenceand targets of interest, the system comprising: means for transmitting(preferably a transmitter) radar signals into a region from an array oftransmitter elements; means for receiving (preferably a receiver) returnsignals of said radar signals returned from within said region via anarray of receiver elements and in a plurality of channels eachcorresponding to at least one of said receiver elements; and means forprocessing (preferably a processor) the return signals to form areceiver beam for each of said channels; wherein the number oftransmitter elements in said array of transmitter elements is greaterthan the number of receiver elements to which each channel corresponds.

Preferably, the system further comprises means for adapting (preferablya adaptor or adaptor module) said signals for transmission from saidtransmitter elements such that said transmitted signals form atransmitter beam which substantially conforms with each said receiverbeam.

Preferably, said adapting means is configured to adapt the signals fortransmission from at least one said transmitter element in a differentmanner than the from at least one further transmitter element.

Preferably, said adapting means is configured to adapt the signals fortransmission from said transmitter elements to form a broadertransmitter beam than would be formed if the signals from eachtransmitter element were substantially the same as one another (forexample in phase and/or amplitude). More preferably, said adapting meansis configured to adapt the phase of the signals for transmission from atleast one said element.

Preferably, said adapting means is configured to adapt the phase of thesignals for transmission from each element in dependence on the positionof the element in the transmitter array.

Preferably, said adapting means is configured to adapt the amplitude ofthe signals for transmission from at least one said element.

More preferably, said adapting means is configured to adapt theamplitude of the signals for transmission from each element independence on the position of the element in the transmitter array.

Preferably, said array of transmitter elements is formed on a surface ofa particular shape, and wherein said adapting means is configured toadapt the signals for transmission from said transmitter elements toform a transmitter beam which is substantially the same as thetransmitter beam that would be formed if the array of transmitterelements were formed on a surface of a different shape.

Preferably, said adapting means is configured to adapt the signals fortransmission from said transmitter elements to form a transmitter beamwhich is substantially the same as the transmitter beam that would beformed if the array of transmitter elements were formed on a curvedsurface (for example, of a cylinder, sphere, or the like).

Preferably, said transmitting means comprises a planar array oftransmitting elements.

Preferably, said transmitting means comprises a non-planar array oftransmitting elements conformal to a known shape.

Preferably, said known shape comprises a shape having a plurality ofplanar facets (for example, a multi-faceted, polyhedral, prismatic,geodesic, and/or pyramidal shape).

Other Method Aspects

According to another aspect of the invention there is provided a methodfor obtaining information about a region including or in the vicinity ofa wind farm, the method comprising: transmitting radar signals into aregion from a location at the wind farm; receiving, at the wind farm,return signals of the radar signals reflected from within the region;and processing the return signals to extract wind farm associated datafor the region.

The radar system may be configured for detecting objects in asurveillance area, the region may be a region within the surveillancearea which has a detection capability which is degraded by wind farmassociated interference, and the return signals may be reflected fromobjects located within the region; and the processing step may compriseextracting wind farm associated data for the objects and analysing thewind farm associated data to enhance detection of the objects within theregion.

The returned signals may comprise indicators of prevailing environmentalconditions in said region; and said processing step may compriseanalysing said extracted data to determine operating parameters for saidwind farm.

According to another aspect of the invention there is provided a methodfor determining operating parameters for a wind farm; transmitting radarsignals into a region from a wind farm, the method comprising: receivingreturn signals of said radar signals reflected from within said regionat said wind farm; and processing the return signals to extract windfarm associated data for said region wherein the returned signalscomprise indicators of prevailing environmental conditions in saidregion; and analysing, in said processing step, said extracted data todetermine operating parameters for said wind farm.

According to another aspect of the invention there is provided a methodfor extracting data in a cluttered environment, the method comprising:transmitting radar signals into a region using a transmitter having afirst aperture; receiving return signals of said radar signals,reflected from within said region, using a receiver having a secondaperture; and processing the return signals to extract data includingclutter related data; wherein said second aperture used in saidreceiving step is of a different size to said first aperture used insaid transmitting step.

According to a further aspect of the invention, there is provided amethod for discriminating between sources of radar interference andtargets of interest, the method comprising: transmitting radar signalsinto a region; receiving return signals of said radar signals returnedfrom within said region; and processing the return signals todiscriminate between return signals returned from a first object andreturn signals returned from a second object wherein said return signalsfrom said second object comprise both zero and non-zero Dopplercomponents and interfere with said return signals from said firstobject; wherein said processing step comprises discriminating betweensaid return signals when said return signals are received at a distancefrom said second object which is less than a proximity limit based onthe geometry of the object.

According to another aspect of the invention, there is provided a methodfor siting a radar system, the method comprising: providing means fortransmitting (preferably a transmitter) radar signals into a region;providing means for receiving (preferably a receiver) return signals ofsaid radar signals returned from within said region; providing means forprocessing (preferably a processor) the return signals to discriminatebetween return signals returned from a first object and return signalsreturned from a second object wherein said return signals from saidsecond object comprise both zero and non-zero Doppler components andinterfere with said return signals from said first object; and sitingsaid receiving means at a distance from said second object which is lessthan a proximity limit based on the geometry of the object.

According to yet another aspect of the invention, there is provided amethod for discriminating between sources of radar interference andtargets of interest, the method comprising: transmitting radar signalsinto a region; receiving return signals of said radar signals returnedfrom within said region; and processing the return signals todiscriminate between return signals returned from a first object andreturn signals returned from a second object wherein said return signalsfrom said objects comprise at least one Doppler component; wherein saidprocessing step comprises discriminating between said return signals independence on a spread of said Doppler components for each object.

According to a further aspect of the invention, there is provided amethod for detecting and tracking an airborne object the methodcomprising: transmitting radar signals in an transmitter beam directedupwardly at no less than 45° relative to a horizon; receiving returnsignals of said radar signals returned from an airborne object withinsaid upwardly directed transmitter beam; and processing the returnsignals returned from said airborne object thereby to detect and tracksaid airborne object.

According to another aspect of the invention, there is provided a methodfor discriminating between sources of radar interference and targets ofinterest, the system comprising: transmitting radar signals into aregion; receiving return signals of said radar signals returned fromwithin said region via an array of receiver elements and in a pluralityof channels each corresponding to at least one of said receiverelements; processing the return signals to form (or represent)concurrently a plurality of beams in the frequency domain, the pluralityof beams comprising at least one beam for each channel.

According to a further aspect of the invention, there is provided amethod for discriminating between sources of radar interference andtargets of interest, the system comprising: transmitting radar signalsinto a region from an array of transmitter elements; receiving returnsignals of said radar signals returned from within said region via anarray of receiver elements and in a plurality of channels eachcorresponding to at least one of said receiver elements; and processingthe return signals to form a receiver beam for each of said channels;wherein the number of transmitter elements in said array of transmitterelements is greater than the number of receiver elements to which eachchannel corresponds.

Other Aspects

According to another aspect of the invention there is provided a radarsystem comprising: a transmitter to transmit radar signals into aregion; a receiver to receive return signals of said radar signalsreflected from within said region, wherein said transmitter and receiverare adapted for location on a structure at a wind farm; and a processorto process the return signals to extract wind farm associated data forsaid region.

The radar system may be configured to detect objects in a surveillancearea. The region may be a region within said surveillance area, whichregion has a detection capability which is degraded by wind farmassociated interference. The return signals may be reflected fromobjects located within said region. The processor may be configured toextract wind farm associated data for said objects and to analyse saidwind farm associated data to enhance detection of said objects withinsaid region.

The returned signals may comprise indicators of prevailing environmentalconditions in said region, and said processor may be configured toextract wind farm associated data for said indicators, and to analysesaid data to determine operating parameters for said wind farm.

According to another aspect of the invention there is provided a radarsystem comprising: a primary receiver to receive radar signals reflectedfrom an object of interest within a surveillance area; a secondaryreceiver to receive radar signals reflected from said object when saidobject is located within a region within said surveillance area; and aprocessor to process said signals received by said primary receiver todetect said object within said surveillance area; wherein said processoris configured to process said signals received by said secondaryreceiver to enhance detection within said region.

According to another aspect of the invention there is provided a radarsystem comprising: a transmitter to transmit radar signals into aregion; a receiver to receive return signals of said radar signalsreflected from within said region, wherein said transmitter and receiverare adapted for location on a structure at a wind farm; and a processorto process the return signals to extract wind farm associated data forsaid region; wherein the returned signals comprise indicators ofprevailing environmental conditions in said region, and said processoris configured to extract wind farm associated data for said indicatorsand to analyse said data to determine operating parameters for said windfarm.

According to another aspect of the invention there is provided a radarsystem for location in a cluttered environment, the radar systemcomprising: a transmitter to transmit radar signals into a region, saidtransmitter having a first aperture; a receiver to receive returnsignals of said radar signals reflected from within said region,reflected from within said region, said receiver having a secondaperture; and a processor to process the return signals to extract dataincluding clutter related data; wherein said second aperture is of adifferent size to said first aperture.

In further aspects, the invention may comprise one, some or all of thefollowing features: a radar located in a cluttered environment; a radarcapable of discriminating an object that has both zero and non-zeroDoppler components; a (preferably holographic) radar operable under theholographic limit; and/or a radar capable of discrimination in a highclutter environment, for example where the clutter is more significantor gives greater returns than likely targets of interest, or where thereturn signals from the clutter would otherwise obscure return signalsfrom targets of interest.

The cluttered environment preferably includes one, some or all of thefollowing: an individual wind turbine (whether off- or on-shore), a windfarm, a collection of wind farms, a ship or groups of ships, seaclutter, buildings and other similar major structures, especially ports,marinas or harbours.

A preferable embodiment of the invention also provides a computerprogram and a computer program product for carrying out any of themethods described herein and/or for embodying any of the apparatusfeatures described herein, and a computer readable medium having storedthereon a program for carrying out any of the methods described hereinand/or for embodying any of the apparatus features described herein.

A preferable embodiment of the invention also provides a signalembodying a computer program for carrying out any of the methodsdescribed herein and/or for embodying any of the apparatus featuresdescribed herein, a method of transmitting such a signal, and a computerproduct having an operating system which supports a computer program forcarrying out any of the methods described herein and/or for embodyingany of the apparatus features described herein.

A preferable embodiment of the invention extends to methods and/orapparatus substantially as herein described with reference to theaccompanying drawings.

Any feature in one aspect of the invention may be applied to otheraspects of the invention, in any appropriate combination. In particular,method aspects may be applied to apparatus aspects, and vice versa.

Furthermore, features implemented in hardware may generally beimplemented in software, and vice versa. Any reference to software andhardware features herein should be construed accordingly.

According to the present invention, there is provided a radar system asset out in the corresponding independent claims. Other preferablefeatures of the invention are recited in the dependent claims.

The invention will now be described by way of example only withreference to the attached figures in which:

FIGS. 1( a) and 1(b) illustrate, in simplified plan, the fields of viewassociated with two different types of radar.

FIG. 2 shows a continuously-sampled time-domain signal for turbine blade(flashes) and a target;

FIG. 3 shows a chopped signal from a target and turbine;

FIG. 4 shows a spectrum of the chopped signal aliased across the entireband;

FIG. 5 shows a chopped signal after filtering;

FIGS. 6( a) to 6(c) illustrate an embodiment of holographic radar;

FIG. 7 shows a high-resolution (holographic) spectrum of a target andturbine;

FIG. 8 shows a target signal recovered by filtering from the holographicspectrum;

FIG. 9 shows an example of an enhanced radar system including anembodiment of holographic radar;

FIG. 10 shows a possible arrangement for the enhanced radar systemaccording to FIG. 9 and the associated azimuth field of views;

FIG. 11 shows the radar system arrangement of FIG. 10 and an associatedfield of views;

FIG. 12 illustrates the use of the radar system of FIG. 9 todiscriminate against wind turbines;

FIG. 13 shows another example of an enhanced radar system including anembodiment of holographic radar;

FIG. 14 shows a simple static sensor array suitable for use inembodiments of the radar system as described herein;

FIG. 15 illustrates a further embodiment of holographic radar;

FIGS. 16( a) and (b) illustrates beam broadening using the embodiment ofFIG. 15;

FIGS. 17( a) and (b) respectively illustrate Doppler-range andDoppler-time characteristics for different targets;

FIG. 18 illustrates a further embodiment of holographic radar;

FIG. 19 illustrates a further embodiment of holographic radar;

FIG. 20 illustrates a further embodiment of holographic radar;

FIG. 21 shows a simplified functional block diagram of a first exemplaryreceiver for use with an embodiment of holographic radar as hereindescribed; and

FIG. 22 shows a simplified functional block diagram of a secondexemplary receiver for use with an embodiment of holographic radar asherein described.

In air traffic control (ATC) and air defence radar systems and the likethe radar transmitter typically scans a volume of interest (eithermechanically or electronically). Thus, in scanned radar systems, targetsare illuminated successively as a transmitter beam sweeps or switchesits position. This has the effect of ‘chopping’ the received signal fromany target into a number of short sequences, with the result (inherentin Fourier-domain signal theory) that changes in target position betweenilluminations result in aliasing of Doppler returns, and that complextracking methods are required in any attempt to discriminate betweenclutter and targets.

FIG. 1( a) shows a field of view for a scanning type radar. The scanningradar has a relatively narrow field of view and has to be swept to allowa large volume of interest to be illuminated, piecewise, in a sequentialmanner thereby effectively ‘chopping’ the signals received from thevolume of interest at a rate determined by the sweep frequency.

FIGS. 2 to 5 illustrate the effect of ‘chopping’ the return signals fromturbine blades (Tb) and targets (Tg).

In FIG. 2 a continuously-sampled time-domain signal is shown in whichturbine blade (Tb) (flashes) and target (Tg) characteristics are bothexhibited. As seen in FIG. 2 movement of the turbine blades (Tb) ischaracterised by six short high amplitude ‘flashes’ (typicallyassociated with a three blade turbine) which, in the example, are verylarge compared with the target signal. The target, on the other hand, ischaracterised by a slow-varying signal which increases and decreases inamplitude as the target approaches and recedes respectively.

FIG. 3 illustrates the ‘chopping’ effect typical of scanning radarsystems for the signal shown in FIG. 2 and FIG. 4 shows a highresolution frequency spectrum for the chopped signal of FIG. 3. Thechopping of the signal effectively represents a significant loss ofinformation about what is occurring in the volume of interest, whichmakes discrimination between target (Tg) effects and turbine (Tb)effects difficult, if not impossible.

As seen in FIG. 4, for example, the chopping of the signal causesaliasing across the entire band. Hence, target (Tg) effects and turbine(Tb) effects cannot be resolved effectively using filtering, and theremoval of the effects of the wind turbine becomes virtually impossible.This is illustrated in FIG. 5, which shows the chopped signal of FIG. 3after filtering, and demonstrates the ineffectiveness of the filteringto remove high-frequency effects associated with the wind turbine. Asseen in FIG. 5, the effects of the wind turbine are still very evident.

In preferred embodiments, a static so-called ‘holographic’ radar is usedwhich is based on the hypothesis that information on the spatialdistribution of objects contained in a particular volume of space can berepresented by electromagnetic illumination from and reception at theboundary of that volume. In broad terms, therefore, three dimensionaldata within a particular three dimensional volume of space can berepresented by two dimensional data at its boundary in accordance withthe holographic principle.

FIG. 1( b) shows a field of view of a simplified embodiment of the socalled holographic radar the principles of which are described below inmore detail with reference to other embodiments. Unlike the scanningradar, the holographic radar of FIG. 1( b) is static, having arelatively large field of view (˜90° or greater) allowing a similarvolume to be illuminated persistently. The radar of FIG. 1( b) has acentrally located transmitter 2 having an associated wide transmitterbeam 2 a and a plurality of receivers 4 each having an associatedreceiver beam (4 a to 4 f).

The holographic radar is configured to illuminate a particular volume ofspace persistently rather than in the discontinuous manner of scanningradar systems. Thus, information contained in signals returned from thevolume being illuminated is not lost as a result of such discontinuity.

All beamforming and direction measurement in the holographic radar isperformed after reception of return signals reflected from within thevolume being illuminated, which effectively allows aliasing effects tobe avoided because above-Nyquist sampling is always available, subjectto a digitally-controlled multiple beamforming strategy, and to certainlimits on the combined range, target speed and operating frequency,known herein as the holographic limit. The holographic limit is definedbroadly as the boundary of the interdependent limits of range, rangerate and operating frequency for which unambiguous operation ofholographic radar can be achieved. The holographic limit may beexpressed mathematically as a bound for which the product of maximumrange (R) and the magnitude of the maximum range rate (dR/dt) must beless than the square of the speed of light (c) divided by eight timesthe operating frequency (F_(op)):

$\left( {{\frac{R}{t}}\left( \max \right) \times {R\left( \max \right)}} \right) \leq \frac{c^{2}}{\left( {8 \times F_{op}} \right)}$

Thus, the use of such a radar also allows the same returns to beanalysed in different ways (for example by the formation multiple beams;some to detect airborne targets without interference from sea clutter;others to assess the sea clutter and surface targets) to extractinformation of relevance to different applications.

FIGS. 2, 7 and 8 illustrate further advantages of using the holographicradar. The holographic radar effectively reproduces thecontinuously-sampled time-domain signal shown in FIG. 2 without the lossof information inherent to scanning radar systems.

In FIG. 7 for example a high-resolution (holographic) spectrum is shownfor the target (Tg) and turbine (Tb) of FIG. 2. Unlike thecharacteristics exhibited in FIG. 4, however, the turbine spectrum isconfined near the upper and lower limits with no artifacts at otherlocations. Thus, as illustrated in FIG. 8, the target signal may beeffectively recovered by filtering from the holographic spectrum.

Holographic Radar Implementation

FIGS. 6( a) to 6(c) illustrate an exemplary embodiment of ‘holographic’radar generally at 200. The holographic radar 200 comprises at least onearray 210 of transmitting elements 214 configured to illuminate a wholevolume of interest simultaneously, with a coherent signal modulatedappropriately (for example as a regular sequence of pulses) to permitrange resolution. It will be appreciated that although an array ofelements is described the transmitter may comprise a single transmittingelement.

The radar includes a control module 250 configured for controlling thesignals transmitted via the transmitter array 210 in dependence on thenature of the application for which the holographic radar is to be used.

The holographic radar also includes a receiving array 220 comprising aplurality of receiving sub-arrays 222. Each sub-array 222 comprises aplurality of receiving elements 224 configured over an area. Eachelement 224 and sub-array 222 of the receiving array is arranged toreceive signals returned from substantially the whole of the illuminatedvolume, each element 224 and/or sub-array 222 essentially forming asignal channel. The receiving array 220 contains more elements than thetransmitter array 210 and has a substantially larger total aperture.

As seen in FIG. 6( a) the transmitter array is ten transmitting elements214 high by two wide. Similarly, each receiver sub-array 222 is tenreceiving elements 224 high by two wide arranged to have substantiallythe same aperture size as the transmitter array. The sub-arrays arearranged in a grid which is two sub-arrays high by eighty wide.

The elements of the sub-arrays 222 are also configured to form further,overlapping sub-arrays 222′, 222″ (or ‘virtual’ sub-arrays) asillustrated in simplified array of sub-arrays (two high by three wide)shown in FIG. 6( b).

As shown in FIG. 6( b) the elements of vertically adjacent sub-arraysare configured to form vertically overlapping sub-arrays 222′.Specifically, the lower five elements in each of the two columns formingeach upper sub-array, and the upper five elements in each of the twocolumns forming each lower sub-array, form a vertically overlappingsub-array 222′ which shares some receiving elements with both theassociated upper and lower sub-arrays.

The elements of horizontally adjacent sub-arrays are similarlyconfigured to form horizontally overlapping sub-arrays 222″.Specifically, for each pair of horizontally adjacent sub-arrays, theelements of the rightmost column of the left sub-array, and the elementsof the leftmost column of the right sub-array, form a horizontallyoverlapping sub-array 222″ which shares some receiving elements withboth the sub-arrays of the associated horizontally adjacent pair.

In the case of the receiver array shown in FIG. 6( a), therefore, thereceiving elements 224 and sub-arrays 222 are arranged in a spacedrelationship, facing substantially the same direction, to form anoverall aperture comprising eighty-two non-coincident, but overlappingsub-apertures in width and three non-coincident, but overlappingsub-apertures in height.

It will be appreciated that although specific array and sub-arraydimensions are described, any appropriate arrangement of sub-arrays andreceiving/transmitting elements may be used depending on therequirements of the application for which they are used. This includes,for example, arrangements having different sub-aperture overlaps (or nooverlaps), different aperture dimensions, arrays which are wider thanthey are tall etc.

Whilst the receiving array 220 (and sub-arrays 222) shown in FIG. 6( a)are planar, it will be appreciated that they may be conformal to someother known shape. It will be further appreciated that each transmittingarray 210 (or element 214) may form part of the receiver array (orpossibly a receiver sub-array).

The radar comprises a signal processing module 240 (such as a computerprocessor or the like) configured such that signals which are coherentwith the transmitted signal may be introduced and used to determine theamplitude, frequency, delay and phase of all signals received at eachelement or sub-array. The processor module is also configured for theformation of multiple beams by combination of different signal channelswith suitable amplitude and phase weightings. The processing module 240is configured for performing the signal processing tasks required by theapplication for which the holographic radar is to be used, for example,beamforming, range gating, Doppler processing, low threshold detection,target tracking (e.g. XYZ, Vxyz, A/Phi(m, n, t)), imaging and/orclassification.

The processing module 240 and the control module 250 may form part ofthe same processing apparatus configured to control radar signalstransmitted by the transmitter array and to process return signalsreceived by the receiving array.

The arrangement of sub-arrays 222 and receiving elements 220 allowsmultiple (overlapping) beams to be formed (e.g. one for each of thedifferent sub-arrays 222), by the processor, which have substantiallythe same look direction, thereby permitting monopulse sub-beam angularmeasurement. The use of a plurality of sub-arrays 222 permits greatermeasurement accuracy than phase monopulse angular measurement using, forexample, a single array of closely spaced receiver elements.

It will be appreciated that although phase monopulse angular measurementis described, the receiver elements and/or receiver sub-arrays may bearranged to allow amplitude monopulse angular measurement.

It will be further appreciated that a plurality of sub-arrays may alsobe arranged having different look directions, effectively creating aplurality of separate apertures/sub-apertures. The sub-arrays may bearranged, for example, to provide a wider (for example 360°) coverage.

Similarly, the sub-arrays may be arranged to look at a known man made orother obstacle from (slightly) different locations thereby allowingtargets which would otherwise be obscured by the obstacle to be resolvedwith greater accuracy. If, for example, the radar is located at a windfarm the receiver sub-arrays may be arranged to ‘look around’ turbineswhich otherwise obscure part of their individual fields of view.

Thus, whilst beamforming on transmission is a process with a single,physically-exclusive outcome, on reception as many beams can be formedas the configuration of the receiver array and the available processingresources can support. Hence, whereas transmission beams must bedirected sequentially, receiving beams may be formed simultaneously.

All targets in the illuminated volume may therefore be illuminated at arate (for example a pulse rate) sufficient to exceed the Nyquist limitfor Doppler shifts associated with all targets, subject to theholographic limit.

In this way the signals from all targets are fully sampled, informationloss is minimised, and alias effects are avoided, resulting in theability to: perform precision tracking and coherent tracking;reconstruct imagery; characterize behaviour; classify targets etc.Tracked targets are represented in computer memory not only in terms ofXYZ and Vxyz as a function of time, but also in terms of the Dopplerphase and amplitude history of the target.

Signals received by the subarrays may simultaneously be combined in afirst data stream such that the amplitude and phase weightings provide anull in the direction of the land or sea surface, to reject surfacetargets, and in a second data stream such that the amplitude and phaseweightings provide a null in the direction of raised objects such aswind turbines, allowing the observation of surface targets and rejectionof turbines or aircraft. Separate filtering and tracking algorithms canthen be applied to the two or more data streams.

The received return signals effectively represent observations (whichmay or may not represent a target of interest) made by the radar in eachof a plurality of signal channels. In the radar, data extracted by theprocessor for each observation is stored in process ‘pixels’. Each pixelmay be considered to be a unique set of numbers representing, forexample, a single combination of time, range, range rate and/or Dopplerfrequency, and at least one of beam number, sub-array number, and/orelement number.

The pixel content comprises a list of numbers representing, for example,a series of amplitudes, phases and/or frequencies representing thecharacteristics of the associated observation, which may constitute atarget, an item of clutter, or a ‘null’, whose behaviour may be deemedinsignificant. Observations are deemed significant until they arereliably known to be insignificant and a history of the information(e.g. phase histories, amplitude histories, or the like) extracted fromthe observations is retained. This approach contrasts with the processof thresholding in which observations are deemed insignificant anddiscarded, without further processing, unless a particular parameter (orset of parameters) meets an associated threshold (or set of thresholds).

In a cluttered environment typified by a wind farm, therefore, the radarfunctions to form the best tracks possible for all targets, based onamplitude and phase histories, and to discriminate between significantand insignificant targets (or targets representing one class or another)at the end of the analysis rather than at the beginning. Coherentanalytical processes can be applied to enhance or minimize targets ofdifferent types, since all signal information is retained within thesignal and target memories.

Such a radar may be limited in its maximum range, either by intent, bylimiting the delays at which signals can be received, or by default,when power, unambiguous range or Doppler coverage are too small, andshould fall within the holographic limit. Such a limited-range sensorprovides a natural basis for an ‘in-fill’ radar within a larger fieldcovered by a standard air traffic control, air defence, marine radar orthe like.

Thus, the configuration of the radar is such that, being holographic inits mode of operation, as described, it is capable of accommodating anddiscriminating targets and clutter in a densely populated environment.

Application as In-Fill Radar

In FIG. 9 a first application of holographic radar within an overallradar service is shown generally at 10. The radar service 10 comprises aprimary radar transmitter/receiver 12 and a secondary radartransmitter/receiver 14. The radar system 10 is configured for scanninga surveillance area 16 which includes a wind farm, another large groupof structures, or the like, which causes interference to transmitted andreflected radar signals thereby resulting in a region 18 of reducedradar performance and hence a degraded detection capability (for exampleas a result of shadowing 20, modulation effects, cascading reflections,or the like).

The primary transmitter/receiver 12 comprises the maintransmitter/receiver of an existing or new radar system (for example thetransmitter/receiver antenna(s) of an existing air traffic controlsystem or the like).

The secondary transmitter/receiver 14 forms part of a holographic radarsystem, generally as described previously, in which the information ontargets contained in a particular volume of space illuminated by theradar can be represented by information within the radar signalsreturned from within that region. The secondary transmitter/receiver 14comprises an array of radar sensors 14′ provided at appropriatelocations for illuminating the region(s) of reduced radar performance.Each radar sensor 14′ is mounted at the wind farm, for example attachedto the tower of an appropriately located wind turbine. Alternatively oradditionally one or more of the radar sensors may be located away fromthe wind farm to create a desired detection field for the secondarytransmitter/receiver 14. It will be appreciated that whilst use of anarray of sensors is advantageous a single radar sensor may be used incertain applications. The radar sensors may be located facing differentdirections from the wind farm and a plurality of such sensors may bearranged at different orientations around a wind turbine to give a widerangle of coverage.

As described previously, each radar sensor 14′ comprises a static sensoras opposed to a rotating antenna, which might cause mechanicalinterference with the turbines. Furthermore static sensors are easier toinstall and are less susceptible to the harsh environment to be expectedat a wind farm especially those sited offshore and subject to severeweather conditions.

The static sensors advantageously comprise arrays of transmitting andreceiving elements (antennas) as generally described with reference toFIGS. 6( a) and 6(b). It will be appreciated however, that in anotherarrangement, the system may comprise a much simpler radar sensor onepossible example of which is described in more detail below withreference to FIG. 13. The ability of the sensor to provide data fordetermining the position of targets is provided for by the relativelocation of the different receiving elements (and/or sub-arrays ofreceiving elements). This allows calculation of phase relationships ordelays between signals received at the different elements (orcombinations of elements) of the receiving array and hence calculationand tracking of position.

Each radar sensor has a wide field of view (typically extending ˜20 km)and can measure directions in both azimuth (typically ˜90° or greater)and elevation (typically ˜20°). Wider angle (for example 360 degree)coverage is provided by installing a plurality of antenna arrays (orsub-arrays), pointed appropriately, or by one or more non-planar arrays.These may be positioned separately, may be located at differentpositions around the perimeter of a turbine support shaft, or may beattached to different turbine supports.

A plurality of sensors 14′ may also be provided which have the same lookdirection but effectively having non-coincident overlappingsub-apertures as described previously. This allows multiple beams to beformed with the same look direction from the different sub-arrays andhence permits monopulse sub-beam angular measurement of greateraccuracy.

A possible arrangement of radar sensors 14′ is shown in FIGS. 10 and 11.The arrangement comprises four sensors; a first of which illuminates afirst region 50 comprising the wind farm itself (or a part thereof); andthe remaining three of which together illuminate a further region(comprising smaller regions 52 a, 52 b, 52 c) extending away from andbeyond the wind farm (relative to the primary transmitter/receiver). Thefirst sensor is configured such that the first region 50 extends notonly in the general direction of the wind turbines but also extends tocover an area above them. Thus, the first sensor persistentlyilluminates the turbines allowing them to be continuously sampled andDoppler resolved. Coherent tracking by the processing apparatus allowsturbine and target returns to be resolved in either range or Doppler.

Each sensor is located relatively low on an associated turbine tower andmay be provided with an upward bias to cover a cylinder of obscurationin three dimensions as illustrated in FIG. 11.

Radar return signals received by the primary transmitter/receiver areprocessed by appropriate processing apparatus 30/32 typically at thelocation of the primary transmitter/receiver (e.g. the air trafficcontrol tower or the like) or distributed between a remote processingcapability 30 at the location of the primary transmitter/receiver and alocal processing capability 32 at the wind farm. The processingapparatus may of course be located exclusively at the wind farm itself.The processing apparatus analyses the return signals to detect and trackobjects of interest as they traverse a surveillance area (detectionfield) of the primary transmitter/receiver into/out of the region ofreduced radar performance.

As seen in FIG. 12 the volume or region of interest may be sub-dividedusing appropriate beam forming and range gating into sub-regions 60,some of which 60′ are occupied by wind turbines (or are at leastaffected by their presence). The sub-division of the volume of interestallows turbine rejection to be applied selectively, thereby minimisingthe possibility of false rejections.

A typical procedure followed by the processing apparatus 30/32 forturbine rejection (e.g. for signals received by the sensor arrayassociated with region 50) will now be described by way of example only.Typically the processing apparatus treats the holographic system aslinear and begins by forming beams for the different sub-arraysincluding beams covering the turbines in the illuminated volume (Beam 1,2, 3 . . . n(turbine(1)) . . . ). Range gates are then formed includingrange gates for the illuminated turbines (RG 1, 2, 3 . . . m(turbine(1)). . . ). A Fast Fourier Transform (FFT) is applied to each rangegate/beam product (RG×Beam) and a high pass Doppler filter is applied tothe resulting FFT for each range gate/beam product associated with aturbine (RG(m)×Beam(n) . . . RG(p)×Beam(q)). A coherent trackingalgorithm (to track phase across the range bins) may then be applied.

In this manner returns from turbines can be effectively rejected. Falsealarm detections are severely restricted and residual dropouts arelimited to slow passes within the range gate and beam.

Radar return signals received by the secondary transmitter/receiver(which are less affected by the presence of the interfering structuresdue to the absence of Doppler aliasing) are appropriately analysed andintegrated with data from the primary transmitter/receiver by theprocessing apparatus to enhance detection and/or tracking of the objectof interest whilst it traverses the region of reduced radar performance.

Data representing targets detected by the secondary radar sensors arecommunicated from the secondary transmitter/receiver to the processingapparatus by suitable communication apparatus 32 for subsequentintegration with data generated by other radar sensors. Typically, forexample, the communication apparatus are configured for wirelesscommunication between the secondary transmitter/receiver and theprocessing apparatus.

It will be appreciated that the secondary transmitter/receiver mayalternatively or additionally be provided with a dedicated processingcapability 34 for carrying out preliminary analysis on the signalsreceived by the secondary radar sensors 14′ before subsequenttransmission to the processing apparatus for further processing of thesignals and integration with data from other sensors. For example,tracks of potential targets of interest may be filtered prior totransmission for processing by the processing apparatus at the primarytransmitter/receiver which is particularly useful for air trafficcontrol systems. Alternatively (or additionally) the raw data may betransmitted for processing by the processing apparatus at the primaryreceiver/transmitter (e.g. for air defence systems).

It will be appreciated that the arrangement of the system allows targettracks to be extracted by the processing apparatus in parallel ifrequired.

Typically the sensor has dimensions of the order of a few meters high bya few meters wide, for example, ˜6 m high by 3 m wide. The sensor istypically mounted on a turbine at a height of between 5 m and 25 m aboveground/high water level, for example, ˜15 m above. The sensors aretypically mounted with an upward bias of a few degrees, for example˜10°. Alternatively the sensor may be mechanically vertical but thebeams electronically steered with an upward bias. It will be appreciatedthat these examples are purely illustrative and that there may besignificant variation in the most suitable implementation used for anyparticular application.

The processing apparatus is also configured for defining a detectionzone for the secondary transmitter/receiver, within the normal detectionfield of that transmitter/receiver, outside which the secondarytransmitter/receiver does not report objects of interest. The detectionzone is defined to be generally coincident with the region of reducedradar performance thereby avoiding unnecessary duplication ofprocessing, activity (for example, by limiting processing of signalsfrom the secondary receivers to times when enhancement is required). Theprocessing apparatus may be configured to define separate detectionzones for each radar sensor of the secondary transmitter/receiver whichmay overlap or may comprise distinct non-overlapping regions. Definitionof detection zones is described in more detail below.

Application in Pre-Emptive Environmental Condition Detection

A further application of such a radar is illustrated in FIG. 13 whichshows a radar system 110 in which pre-emptive measurement of wind fieldcan be made in preparation for wind energy generation or in real time aswind eddies or the like approach a wind farm or individual wind turbine.In this case precipitation, eddies and turbulence in the wind cause thescattering of radar signals that can be detected, leading to imagery ofthe wind field itself.

The radar system 110 comprises a transmitter/receiver array 114′ locatedat a wind farm generally as described previously with the transmitterelement(s)/array(s) arranged for illuminating a region 116 of interesteither around the wind farm as shown or in a particular direction ofinterest.

The system 110 includes processing apparatus 132 configured forprocessing signals returned from within the illuminated volume toextract indicators of prevailing environmental conditions 140 within thevolume at some distance from the wind farm. These indicators areanalysed by the processing apparatus and the relevance of theenvironmental conditions 140 which they signify, to the wind farm, isdetermined. The processing apparatus determines appropriate (optimal)operating parameters for the wind farm (or one or more individualturbines) based on this analysis and outputs the parameters for use incontrolling the wind turbines. The output may include timing data toindicate a time at which the current operating parameters of the windfarm should be modified based on a prediction of when the environmentalconditions of interest will reach the turbines.

The processing apparatus may, for example, determine that specificenvironmental conditions 140 are approaching a wind-farm from somewherein the illuminated volume and that the conditions are of a type (such aschanges in the characteristics of the wind) relevant to the wind farm'soperation (e.g. its efficiency, its generation capability, itsrobustness and safety, or the like). Hence, based on this analysis, theprocessing apparatus provides wind (or other environmental) parametersappropriate to determine operating parameters of the wind farm (or oneor more individual turbines) for when the approaching environmentalconditions arrive.

The radar system 110 may comprise a controller for adjusting theoperating parameters of the wind farm, individual turbines, and/orindividual turbine components (e.g. blades) or may be configured forcompatibility with an existing controller at the wind farm. Accordingly,the processing apparatus is configured for outputting operatingparameters in a suitable format for interpretation by the controller. Itwill be appreciated that whilst automatic control of the turbines inresponse to predictions of changing environmental conditions isdesirable, the output from the processing apparatus may include visualor audible signals for interpretation and implementation by an operatorof the wind farm.

The operating parameters may comprise any modifiable attribute of thewind farm including, for example, the angle which a turbine faces, thepitch of each blade, the status of each blade (e.g. feathered oroperational), the status of each turbine (e.g. operational orshut-down), and/or the like.

The environmental conditions may comprise any radar detectableconditions including, for example, wind characteristics 142 (e.g. windspeed, wind shear, turbulent eddies, or the like), precipitation/airborne moisture characteristics 144 (e.g. density, type, etc.), and/orany surface effects 146 (such as wave formations for off-shorefacilities).

The mean air flow past a wind turbine affects the available energy,according to a substantially cubic dependence. However the efficiencyand safety of a wind turbine is related to variations in the wind speed.A high wind speed near the top of the turbine (assuming a horizontalspin axis), and a lower wind speed closer to the surface (whether theground or the surface of the sea) can lead to inefficiency given thatthe upper and lower blades move at the same speed. This can be mitigatedby modifying the pitch of the blades during rotation between upper andlower positions. Hence, the processing apparatus is configured to takeaccount not only of the overall environmental conditions in the regionof interest (for example average wind speed) but also to take account oflocalised differences (for example stratified wind speeds, and/or thoseresulting from turbulence) in the weather patterns.

When a turbulent burst impinges on the turbine the blades may beaffected in such a way that stresses build up in directions that can notbe accommodated by circular motion of the airfoil. This may represent ahazard to the turbine or its surroundings, and could result in itsdestruction. In that case it may be beneficial to ‘feather’ the blades,or otherwise modify their pitch in time, which may be possible if someseconds' advance warning of the burst is available.

The indicators of the environmental conditions may be associated withany environmental features which cause radar reflections. For example,when air moves across the land or sea surface, wind shear and turbulenteddies are generated. Hence, the flow of precipitation (or suspendedmoisture) in the air is also modified by the air motion. As the densityof air or windborne precipitation (or suspended moisture) varies, radarsignals propagating through the air are scattered or reflected and maybe received by a radar receiver. These signals may be detected andinterpreted in terms of the air flow using the principles outlinedabove.

Whilst scattering in such environments is weak and occurs in thepresence of larger, unwanted targets such as land or sea clutter,holographic radar as described may achieve the necessary sensitivity bysufficient coherent integration of scattered signals, and may be used todiscriminate between the wind motion and the surface clutter in eithercase. Wind flow, for example, tends to be vertically stratified, andreflections at different layers may be resolved either by verticalreceiver beamforming or by observing their different Doppler frequenciesand measuring their phases across the receiving array.

Holographic radar located near a wind generation system can thereforeallow wind patterns to be measured beneficially, either to assess thepotential wind energy resource, or to enable improved control and safetyof one or more installed wind turbines.

In a further environmental application of the holographic radar, theradar's vertical beam pattern may be digitally re-formed to achieve ahigh sensitivity in the plane of the sea surface, rather than theminimum sensitivity required in the case of wind imaging against seaclutter. In this case the signals generated by waves are observed ratherthan rejected, and the holographic radar may then be used as a wavesensor.

In each application, the fact that data from all observations isretained and processed, rather than discarded according to specificthresholds, means that the applications can be implemented in parallelwith the processing apparatus configured to analyse the stored wind farmassociated data purposively in dependence on the requirements of thespecific application. For example, information on environmentalconditions may be considered irrelevant to the augmentation or ‘in-fill’application and so may be ignored for the purposes of improving thedetection capability of an air traffic control, air defence and/ormarine radar system. Contrastingly, for the purposes of ensuring optimumoperating conditions at the wind farm, this same environmentalinformation may be extracted and analysed whilst information on movingobjects such as aircraft, ships, or the like is ignored.

Antenna Arrays

With reference to FIG. 14, there is shown a diagrammatic representationof a planar antenna array 300 suitable for use as a static sensor or thelike in an embodiment of this invention.

The antenna array 300 is constructed on a (preferably insulative)substrate 302. The substrate may be a block of plastic or glassfibrecomposite material (or similar) having a flat supporting surface. Inorder that embodiments of the invention are available for use wherespace is restricted, the antenna array is compact, having a peripheralsize depending on the arrangement of antenna. For example, the array maycomprise an offset transmitter antenna and a trapezoidal/triangularreceiver array, with the transmitter element comprising a four by twosub-array and each receiver element comprising a two by two sub-array.Alternatively the array may be arranged with a central transmitter andfour peripheral receivers. Antenna elements are formed on the supportingsurface of the substrate as conductors printed onto the surface. Theantenna elements may be dipoles (for example, bow-tie dipoles), TEMhorns, microstrip patches, stacked patches, or any other compact elementor conductive structure suitable for operating at the required signalfrequency. Such an antenna is described in WO01/059473; and is generallybest suited for short range applications in which high sensitivity isless important and in which fewer targets are expected within thedetection field.

For applications requiring higher sensitivity and the ability to resolvemore targets a higher-power transmitter array and a larger receiverarray may be required, each of whose sub-arrays has the same field ofview as the transmitter array.

It will be appreciated that the elements/sub-arrays may not be mountedon a common substrate for mechanical robustness. In such an arrangementhowever, the elements/sub-arrays may still be mounted in a common plane.

In the example of FIG. 14, the array 300 has four antenna elements intotal. Three of these elements are first, second, and third receivingelements 304, 306, 308 although other numbers of receiving elements,such as two, three, five or more, may be provided. The fourth element isa transmitting element 312. The receiving elements 304,306,308 aredisposed at the vertices of a triangular shaped (which may, in a specialcase be a right angled triangle) locus, and with more elements thesecould be disposed at the vertices say of a trapezoid or an irregularplanar locus. In the case of a three-dimensional substrate they may beat the vertices of a cuboid or other solid form. The transmittingelement 312 is disposed at the centre of the same locus.

Subject to the requirements for sensitivity and resolution the size ofthe antenna array is preferably kept to a minimum. For example, in thisminimum case, the spacing between the elements may be in the order of nomore than a few half-wavelengths. For example at an operating frequencyof 6 GHz, spacings may be a few centimetres, say between 1 and 10 cm,preferably between 2 and 8 cm.

In an alternative form of construction, the antenna elements may belocated within a dielectric radome. Associated signal processingcircuitry may also be located within the radome in order to provide theapparatus as a self-contained package. It will be appreciated, however,that the array may comprise any suitable configuration, and whereaccurate positional information is not required may comprise a singlereceiver antenna.

Defined Detection Zone

As described above, in the case of the detection augmentationapplication, the processing apparatus may be configured to sub-dividethe detection field of the secondary transmitter/receiver into a firstzone in which detection events are considered to be significant for thepurposes of enhancing radar performance (a detection zone), and a secondzone in which such radar enhancement is not required. Similarly, for thepurposes of environmental condition analysis the processing apparatusmay be configured to sub-divide the detection field into a plurality ofdifferent zones, for example for which different types of analysis maybe performed (e.g. to allow the effects of different physical surfacefeatures such as localised land (or water) masses or environmentalconditions to be catered for). The sub-division may be carried out byany suitable means, typically by a software program based on combiningsignals from different sub-arrays with appropriate phase and amplitudeweightings.

The processing apparatus may operate to execute an algorithm thatdefines a 3-dimensional volume of space within the detection field asthe detection zone. For example, the detection zone may be defined tolie between spaced planes by specifying that it is bounded by minimumand maximum values of X, Y, and Z ordinates in a Cartesian coordinatesystem within the detection field of the array. Alternatively, thedetection zone may have an arbitrary shape, defined by a look-up tableor a mathematical formula. Thus, the detection zone can havesubstantially any shape that can be defined algorithmically, and canhave any volume, provided that it is entirely contained within thedetection field. Flexible detection zone definition such as this isparticularly advantageous in defining a zone in which enhancement isrequired because the regions of reduced radar performance may be subjectto change, for example as new wind turbines are added to existing farmsor as existing turbines are switched out of operation for maintenancepurposes. Similarly, in the case of environmental condition detection,the flexibility allows a zone to be set-up to track a particularenvironmental feature of interest (e.g. a storm, squall or microburst).

In the case of radar augmentation, the processing apparatus may beoperative to detect and track entry of an object of interest into andthrough the detection zone and to integrate data from both the secondaryand primary transmitter/receivers thereby to enhance detection withinthe region of reduced radar performance.

As a development of this embodiment, the processing apparatus may definea plurality of detection zones. The detection zones may benon-coextensive (overlapping, separated or spatially different) and/oralternatively defined, by which it is meant that differentcharacteristics are used for determining whether an object (or aparticular environmental condition) is in the relevant detection zones.For example, different zones may be provided for detecting differentspeeds or different sizes of objects. This can, for example, be used toensure continuous tracking of objects of different sizes and/or speedsinto and out of the region of reduced performance.

In another development of this embodiment, the processing apparatus isoperative to analyse characteristics of objects (or prevailingenvironmental conditions) outside of the detection zone. Suchcharacteristics may be, for example, size of the object, distance of theobject from the apparatus and/or the detection zone, direction ofmovement of the object relative to the apparatus and/or the detectionzone, and relative speed of the object. As an example, the processingapparatus may be operative to track objects outside the detection zoneand to predict their entry into the detection zone. It will beappreciated that such functionality is equally beneficial for monitoringthe movement of environmental features of interest relative to thedefined detection zone(s).

A further embodiment is summarised below by way of example only.

The embodiment is directed to augmenting the function of surveillanceradar systems in the presence of new structures, for example toameliorate the effect of new structures, and in particular wind farms onair traffic control radar systems.

The embodiment comprises the combination of a suitable form of radarsensor or sensors with the method of enhancing air traffic control bymounting them at the wind farm itself for example attached to theturbines.

Each radar sensor is one that does not require a large, rotatingantenna, thereby avoiding mechanical interference with the turbines. Astatic sensor will be easier to install and less susceptible to theharsh environment to be expected at the wind farm. Many wind farms aresited offshore and are subject to severe weather.

A preferred implementation of the radar sensor is one using static(preferably asymmetric) arrays of transmitting and receiving elementswhose region of illumination is the whole field of view and whoseregions of sensitivity may be selected within that field. Their abilityto measure the position of targets is provided by calculation of phaserelationships or delays between signals received at different elementsor combinations of elements of the receiving array.

Each radar sensor has a wide field of view and can measure directions inboth azimuth and elevation. 360 degree coverage is provided byinstalling more than two planar antenna arrays, pointed appropriately,or by one or more non-planar arrays. These may be positioned separately,may be located at different positions around the perimeter of a turbinesupport shaft, or may be attached to different turbine supports.

Data related to targets detected by the radar sensors are communicatedpreferably by a wireless or optical link to the user, the air trafficcontrol/air defence system or systems and integrated by suitablecomputer software with the data generated by other radar sensors.

Broad Beam Transmitter

As described for the holographic implementation above, the transmitterilluminates the whole volume of a field of view substantially (or indeedentirely) simultaneously. The receiver array of the holographic radarmay comprise sub-arrays having any suitable arrangement of receiverelements, each having appropriate dimensions. Accordingly, thesub-arrays may be very small comprising only a few elements or even asingle element. For example, as shown in and described with reference toFIG. 14 the receiver array may even comprise a plurality of individualreceiver elements (each of which can be thought of as equivalent to a‘sub-array’ comprising a single receiving element). Each receiverelement or subarray receives signals from the same volume andbeamforming and monopulse algorithms are applied to resolve thepositions of targets.

In order to provide transmitter beam patterns which correspond to thefield of view for each receiver sub-array (or element) as shown in anddescribed with reference to FIG. 1( b), the transmitter may be designedto have a transmitting antenna aperture which is smaller than that ofthe receiver array but equal to that of the sub-array. Accordingly, inthe holographic implementation described with reference to FIGS. 6( a)to 6(c) above, the transmitter array comprises the same number oftransmitter elements as the receiver sub-array. Therefore, where thereceiver sub-arrays are small (or where the receiver comprises aplurality of individual receiver elements) the transmitter comprisescorrespondingly few transmitter elements (or even a single transmitterelement) through which all the transmitted power must be radiated. Thisresults in a significant demand being placed on the transmittingcircuitry in the control module 250 (including, for example, the signalgenerator amplifier, and/or power combining networks).

Another advantageous exemplary embodiment of a holographic radar inwhich the demand on transmitter circuitry may be reduced is shown inFIG. 15 generally at 400. The holographic radar 400 comprises at leastone array 410 of transmitting elements 414 configured to illuminate awhole volume of interest simultaneously generally as describedpreviously. The radar includes a control module 450 configured forcontrolling the signals transmitted via the transmitter array 410 independence on the nature of the application for which the holographicradar is to be used.

The holographic radar also includes a receiving array 420 comprising aplurality of individual receiving elements 424. Each element 424 of thereceiving array is arranged to receive signals returned fromsubstantially the whole of the illuminated volume, each element 424essentially forming a signal channel. The receiving array 420 containsmore elements than the transmitter array 410 and has a substantiallylarger total aperture. The receiver array is provided with signalprocessing module 440 as generally described previously with referenceto the signal processing module 240 in FIGS. 6( a) and 6(c).

In this embodiment, the transmitter array comprises an extended arrayhaving a plurality of antenna elements and, accordingly, comprises agreater number of transmitter elements than the single receiver elementused for each receiver signal channel. Using the extended array helps tomitigate the demands on the transmitting circuitry required for eachtransmitter element, when compared to use of a single transmitterelement. It will be appreciated that although a square three by threearray of transmitting elements is shown the extended transmitter arraymay comprise any suitable number of transmitting elements in anysuitable arrangement. For example, the transmitter array may comprise asmany as 20, 50, 100 or even more transmitting elements arranged in asquare, rectangular or other appropriate shape array.

Generally, as a skilled person would understand it, an extended array oftransmitter elements would inherently result in a narrower transmitterbeam than that of each receiver sub-array (or element) as illustrated inFIG. 16( a) and, accordingly, transmitter beam patterns which do notcorrespond to the field of view associated with each receiver element.

In order to allow transmitter beams to be generated which correspond (orvirtually correspond) with the broad field of view desired for eachreceiver element, the control module in this embodiment is configured,with the antenna element interconnections, to control the phases and/oramplitudes of the radar signals transmitted by the transmitter elementsforming the extended array. More specifically, the control module isconfigured to adjust the phases and/or amplitudes of the signals totailor the transmitter beam (for example in a progression across thearray) to form the desired beam pattern.

As shown in FIG. 16( b) for example, the phase of the signalstransmitted from a planar array of transmitter elements may be adjustedto approximate a broader beam pattern that would be expected if thetransmitter elements were located on a smooth curved surface such as acylinder, sphere or dome. The amplitude of the transmitted signals maysimilarly be adjusted to further tailor the beam pattern and, inparticular, to mitigate edge effects such as side lobe formation bytapering the signal amplitude toward the edge of the extendedtransmitter array.

Thus, in this embodiment, an extended ‘multi-element’ transmitting arrayis used but the amplitudes and phases of the elements are adjusted (orcontrolled explicitly) to generate a widely diverging beam instead of anarrow beam. Hence, in the transmitter circuitry, different sub-circuitsmay be used for each transmitting element and their combined power isradiated over the wide field of view. This mitigates the need either fora single, very high-power transmitting circuit, or for combining thepower outputs of many transmitter sub-circuits into a single feed (whichwould result in associated losses).

In a variation of this embodiment the transmitter elements may bearranged in a non-planar configuration on the planar facets of apolyhedral surface approximating the curved surface which the phaseadjustments are intended to mimic. In this case the phase (andamplitude) modifications required to produce a broad beam pattern (andmitigate edge effects) corresponding to that of the receiver elementscan be simplified when compared to a planar transmitter array. The useof a polyhedral shape has the advantage that it is easier to fabricatethan a smooth curved surface and therefore represents a good compromisebetween a planar transmitter array which requires relatively large phaseadjustments and the relatively costly fabrication of a smooth curvedtransmitter surface. The structure could, for example, be anyappropriate polyhedral shape such as a prismatic, pyramidal or geodesicshape.

Discrimination Based on Spread of Doppler Spectrum

Generally, conventional systems directed to mitigating the effects ofmoving clutter, such as wind turbines, on radar capability treat windturbines as objects which effectively cannot be classified in their ownright. Such systems are generally designed to reduce the effects of windturbines on radar returns from objects of interest such as aircraftwithout actually tackling the root cause of the problem; an inability toeffectively identify return signals originating from wind turbines andthereby separate them from return signals originating from otherobjects.

The implementation of a holographic radar system (as describedpreviously) at or in the vicinity of a windfarm (either as a stand alonesystem or as an in-fill radar for a larger surveillance system),however, provides the possibility of significantly improvingcapabilities for actively discriminating between radar returns from windturbines and radar returns from other objects of interest such as, forexample, aircraft.

Accordingly, in another exemplary embodiment of the invention theholographic radar (implementations of which are described in more detailelsewhere) is configured to discriminate between signals returned from awind turbine (or similar) and those returned from other targets. Asdescribed previously, the holographic radar is configured to operatewithin the holographic limit and, accordingly, is capable of measuringthe full Doppler spectrum of a target with a resolution which depends onthe observation interval.

In this embodiment the holographic radar is configured to carry out 10observations a second and to measure a Doppler spectrum extending to amaximum Doppler frequency in the region of 1 kHz with a resolution ofapproximately 10 Hz. Thus, in operation, the Doppler spectrum measuredby the holographic radar in this embodiment will comprise approximately100 Doppler bins into which the Doppler spectrum of return signals maybe divided. It will be appreciated, however, that the holographic radarmay be configured to make observations using any suitable observationinterval, over any suitable range of Doppler frequencies (subject to theholographic limit), and may be operable to measure a Doppler spectrumover any suitable range using any appropriate frequency resolution. Insome applications, for example, observations may be made up to 10 kHz oreven greater frequencies and the Doppler spectrum may be split into morethan 100 Doppler bins, for example 200 bins, 800 bins, or even 1000 binsor more.

The holographic radar is also configured for forming a plurality ofrange gates defining a plurality of range bins (e.g. as illustrated inFIG. 12) into which targets detected by the radar may be categorised.

Exemplary Doppler-Range characteristics for different targets areillustrated, by way of example only, in FIG. 17( a) in which the arrowsrepresent the evolution of the Doppler-Range characteristic for thetargets over time. Exemplary Doppler-Time characteristics for thetargets shown in FIG. 17( a) are illustrated in FIG. 17( b). It will beappreciated that FIGS. 17( a) and 17(b) are purely illustrative, and aresimplified. Furthermore, other turbine designs (for example, verticalaxis and/or helical blade designs) may yield significantly differentDoppler characteristics.

As seen in FIG. 17( a), the Doppler spectrum associated with returnsfrom a rotating turbine blade will generally be spread across the entiremeasured spectrum (10 Hz to 1 kHz in this embodiment). The effect of theturbine blade's rotation will therefore be observeable in most if notall 100 Doppler bins substantially simultaneously. Furthermore, sincethe turbine tower does not move in range, the effects of the rotation ofthe turbine blade will generally appear in only a single range bin.

Contrastingly, for the case of a typical radar operating frequency inthe region of 1 GHz, the Doppler spectrum of an approaching orretreating target such as an aircraft will generally appear within onlya single Doppler bin at a time (when subject to manoeuvres at less than1 g (9.81 m/s²)). Furthermore, as seen in FIG. 17( a) as the targetapproaches or retreats from the radar, it will be seen to move in rangefrom one range bin to another.

In FIG. 17( b) the turbines are seen to exhibit turbine ‘flashes’,having Doppler components across the entire Doppler spectrum, as eachblade (of the three blades in the illustrated example) in turn reachesan orientation (in a direction approaching the receiver) which issubstantially perpendicular to the line of sight of the radartransmitter/receiver arrays. At this point the returns from the bladeare instantaneously coherent in phase resulting in a periodic, largeradar cross-section flash. For the rest of the time, when the blades arenot perpendicular to the line of sight, the vector sum of the differentcomponents is destructive, as a consequence of the variability of thephase. It is possible that ‘flashes’ may also be seen as each blade inturn reaches a perpendicular orientation when retreating from thereceiver (shown as thinner lines in FIG. 17( b)) although these willtend to be less powerful, possibly as the result of the trailing bladeedge having a lower radar cross-section than the leading edge.

FIG. 17( b) also shows a theoretical envelope for Doppler componentsseen for each blade between blade flashes. The edge of each enveloperepresents the theoretical Doppler components associated with the tip ofthe associated blade. In theory, Doppler components (for other parts ofthe blade) will be spread throughout the Doppler envelope although inpractice the Doppler components may be more powerful (and hence morevisible) as the blade approaches the receiver than when it retreats.

As seen in FIG. 17( b) the Doppler characteristics appear to ‘swamp’other characteristics such as those shown for the approaching andretreating targets, especially during the turbine flashes. In the caseof scanning radar these characteristics cause the aliasing previouslydiscussed and effectively prevent targets of interest beingdiscriminated from the effects of wind turbines.

The holographic radar in this embodiment, however, is configured to usethe Doppler characteristics, in conjunction with range and historicalinformation (which may comprise Doppler histories, range histories orboth), to discriminate between signals returned from a wind turbine andthose returned from targets of interest such as an aircraft, therebyallowing the wind turbine to be to detected and identified, and thetarget of interest to be detected, identified and tracked in thevicinity of the wind turbine.

The holographic radar of this embodiment is configured to identify atarget in dependence on the spread of the Doppler components it producesacross the Doppler spectrum. Accordingly, a target which appearssimultaneously in more than a predetermined number (or proportion) ofthe available Doppler bins (referred to herein as the ‘Doppler spreadthreshold’) at substantially the same time (and at substantially thesame distance) may be classified as a rotating object (such as a turbineblade). The Doppler spread threshold above which an object is classifiedas a rotating object (such as a turbine blade) may be any suitablenumber (or proportion) of Doppler bins typically, for example, anywherebetween 5% and 100% (e.g. 5%, 10%, 20%, 50%, 80%, 90%, or 95%) of theavailable bins depending on the design of the wind turbines, and theexpected nature of the targets of interest, which require discriminationfrom one another. An object identified and classified as a rotatingobject (such as a wind turbine) in this manner may then be ignored forsubsequent threat analysis.

The holographic radar is also configured to retain and monitorhistorical data for detected targets (e.g. in ‘process pixels’).Discrimination between wind turbines and other targets may therefore befurther enhanced, based on this historical data, by analysing the rangecharacteristics of the detected targets over time. If a targetexhibiting a spread of Doppler above the Doppler spread thresholdappears in a single range bin (or possibly a limited number of rangebins), for example, and remains there for a predetermined number ofobservations, it is identified as a ‘stationary’ (in range) objectexhibiting some form of rotation (e.g. a wind turbine). Contrastingly,if an object appears to move from one range bin to another over time itis unlikely to be a wind turbine regardless of the frequency componentsit exhibits (which may instead be associated with another rotatingobject such as a helicopter blade for example). It will be appreciatedthat changes in the azimuth angle of the object relative to the receivermay be used in a similar manner to discriminate between the windturbines (which remain at substantially the same azimuth angle) and anobject moving tangentially across the holographic radar's field of viewwith little or no radial velocity component.

It will also be appreciated that a wind turbine may appear in more thanone range bin (or at more than one azimuth angle) (e.g. by virtue of themovement of the blades and/or rotation to face the wind) but will notmove beyond a limited selection of ranges (or azimuth angles).Accordingly the classification algorithm may be adapted to take suchsituations into account.

In another version of this embodiment the identification of windturbines is further enhanced by analysing the Doppler spread history ofthe detected targets. In this case not only is a target identified as anobject such as a wind turbine based on the instantaneous spread ofDoppler at a particular time but also on the evolution of the Dopplerspread with respect to time. For example, if a detected target is seento occupy a large number of Doppler bins (e.g. exceeding the Dopplerspread threshold) and then fewer (or even zero) Doppler bins on aperiodic basis it may be identified as a rotating object such as a windturbine. Where the turbines to be viewed by the holographic radar arewell characterised the Doppler evolution based classification algorithmmay be more sophisticated allowing turbines to be identified even moreaccurately. For example, the algorithm may be adapted to identify atarget to be a wind turbine if the set of Doppler frequenciescharacterising the object develop in accordance with a predefinedmathematical model or function (e.g. comprising a sinusoidal,logarithmic, quadratic, and/or exponential, model or function). As afurther example, by comparing the spread of Doppler frequencies (whichis a measure of the speed of the fastest points on the clutter object,or the blade tip for a wind turbine) with the interval of repetition ofthe ‘flashes’, the length of the turbine blade may be inferred.

The holographic radar is also configured to positively identify adetected target as a target of interest (or a potential target ofinterest) if the detected target appears, or consistently appears infewer than a further Doppler spread threshold comprising predeterminednumber (or proportion) of the available Doppler bins (referred to hereinas the ‘Doppler ceiling’). The Doppler ceiling threshold below which anobject is classified as a target of interest (or a potential target ofinterest) such as an aircraft may be any suitable number (or proportion)of Doppler bins typically, for example, anywhere between a singleDoppler bin and 25% of the available Doppler bins (e.g. 1%, 2%, 3% 5%,10%, 20%, or 25%) of the available bins depending in particular on theexpected nature of the targets of interest and also on the nature ofexpected (e.g. wind turbine related) clutter. An object identified andclassified as a target of interest (or a potential target of interest)in this manner may then be subject to subsequent threat analysis.

It will be appreciated that theseclassification/identification/discrimination techniques may be used inconjunction with other such techniques to further enhance the accuracyof discrimination between interfering objects such as wind turbines andtargets of interest and to enhance threat analysis once a target ofinterest is identified. For example, the techniques may be enhancedbased on the elevation and/or azimuth angles at which the targetsappear, the historical position of the target (e.g. the target's track),the direction a target is moving (e.g. the target's trajectory), thetarget's acceleration or the like.

Thus, under these conditions the use of this type of radar (operatingsubject to the holographic radar limit) can provide a potentialimprovement in the region of 100:1 or even better in terms of thedetectability of an aircraft in the presence of a wind turbine or windfarm.

Fresnel Zone Clutter De-Emphasis

As described above, unlike a scanning radar, a holographic radaroperating under the holographic limit can be configured to successfullydiscriminate between wind farm induced radar returns (which may bethought of as wind farm clutter ‘WFC’ or wind turbine clutter ‘WTC’),including returns associated with rotating blades, even when the radaris located within the vicinity of a wind farm. Specifically, theholographic radar is operable to successfully identify and mitigateagainst substantially all wind farm induced clutter, and to successfullydetect and track targets of interest, even at a proximity for whichother radar systems (such as scanning radar) would not be able to detecttargets of interest or would not be able to detect and track them withthe degree of accuracy and consistency required (e.g. for accurate andhence safe air surveillance).

Configuration of a holographic radar to detect and identify radarreturns from interfering objects such as wind turbines when theholographic radar is located at a relatively close proximity to theturbine provides additional surprising secondary benefits. Specifically,location of a holographic radar in the vicinity of a wind turbine hasthe potential to provide unexpected improvements in the accuracy andefficiency with which the returns from the turbine can be discriminatedfrom returns induced by targets of interest, even when compared with asimilar holographic radar located, and configured to operate at, agreater distance from the turbines.

To illustrate the benefits of locating the radar at close proximity, thesituation in which there is a large distance between a target and aradar transmitter/receiver will first be considered. At these distancesthe effective radar cross-section ‘a’ of the target can generally beassumed to be constant with respect to range.

The assumption that radar cross-section remains constant, however, onlyholds when the signals returned from the target exhibit phase deviationsthat are determined by the local geometry of the target rather than bythe radius of curvature of the incident wavefront. At these distancesthe radar waves incident on the target can be approximated as aplane-wave (for which the source would effectively be at infinity) andhence the phase deviation across the entire target will be dominated bythe target geometry.

In the case of wind turbines the effective radar cross-section atlong-distances is very large, partly because the turbine tower and theblades are themselves large, and partly because the beam reflected bythe tower and/or blade is generally very narrow in the plane containingthe reflector. The beam width reflected from the blade, for example, isdependent on the blade's curvature which is generally small therebyresulting in a narrow beam and large effective radar cross-section, andon its length, which defines a narrow diffraction pattern at thewavelength of operation of the radar (which may be between 1 and 30 cm).Hence, the tower and/or blade appear as a high cross-section reflectorat such long distances.

Contrastingly, even though targets that are of interest such as aircraftmay have large features such as fuselage and wing, they must bedetectable when they are at a disadvantageous orientation (e.g. headingtowards the radar). At such orientations the effective radarcross-section of an aircraft can be predominantly determined byscattering from features with smaller radii of curvature (such ascorners between wing and fuselage, engine nacelles, etc.). Hence theeffective cross-section of an aircraft, relative to that of a windturbine, can be very small making it more difficult to identify theaircraft when the radar has to look past a wind farm (or even anindividual turbine) to see it. In such situations conventional scanningradar can become effectively swamped by the returns from the turbines.

When a radar transmitter/receiver is closer to a target such as a windturbine, however, the curvature of the incident wave becomessignificant, the plane-wave approximation is therefore no longerapplicable, and the phase deviation of the returns from a larger targetcannot be assumed to be negligible. When the radar is particularly closeto the target, for example, the returns from across the target begin toexhibit a phase deviation in excess of 180° (half a wavelengthdifference). The distance between a target and the radar at which thisoccurs will be referred to herein as the ‘proximity limit’.

In the case of targets such as aircraft the radar cross-section is muchless sensitive to the curvature of the incident wave even within theproximity limit because the less regular features of an aircraft canmake the radar cross-section much less dependent on distance (and eveneffectively independent of distance). This is because even at relativelyshort distances the radii of curvature of the features themselves can bethe dominant contributor to the radar cross-section.

The proximity limit ‘D_(p)’ may be determined by considering the extentof a hypothetical ellipsoid of revolution (having a circularcross-section sometimes referred to as the First Fresnel zone or simplythe Fresnel zone) extending from the radar transmitter towards a target,and within which a target will exhibit a phase deviation of less than180°. The radius ‘r_(F)’ (the Fresnel radius) of the circularcross-section of the ellipsoid at the target depends on the wavelengthof the transmitted signal ‘λ’ and the distance ‘D’ between the targetand the transmitter as follows:

$r_{F} \approx \sqrt{\frac{\lambda \; D}{2}}$

When a target is at the proximity limit D_(P), therefore, the extent ofthe Fresnel zone at the proximity limit will substantially coincide withthe extent of the target. Thus, for a circular target of radius r_(tg):

$D_{p} \approx {\frac{2}{\lambda}r_{tg}^{2}}$

When a radar is located within the proximity limit the large phasedeviation of the returns effectively causes a reduction in the radarcross-section as seen by the radar receiver.

Accordingly, another embodiment of the holographic radar in which thisprinciple is advantageously applied is illustrated in FIG. 18 generallyat 500. The radar 500 comprises transmitter and receiver arrays 502 andradar control and analysis unit 504 including a transmitter controllerand a receiver signal processor. The transmitter and receiver arrays502, the transmitter controller and the receiver signal processor aregenerally as described for any of the other embodiments and will not bedescribed again in detail.

The radar 500 is configured to illuminate a region including a wind farm506, to receive and analyse signals returned from within the region, andto discriminate between signals returned from wind turbines and signalsreturned from other targets (such as aircraft) generally as describedpreviously. The radar 500 is shown as a standalone radar in FIG. 18 andcan be used as such. It will be appreciated, however, that the radar 500may form part of an in-fill radar system as described earlier.

The radar transmitter and receiver arrays are located at a distance ‘D’from the wind turbines of the wind farm. The distance ‘D’ is selected tobe within the proximity limit of the turbine blades of the furthest windturbine. Specifically, where L_(B) is the length of each blade (assumingall turbines in the farm are of the same size), the distance from thefurthest turbine blade ‘D_(max)’ (at which distance the Fresnel radiusis r_(Fmax)) may be selected based on the following design inequality:

${D_{\max} \leq D_{p}}\therefore{D_{\max} \leq {\frac{2}{\lambda}L_{B}^{2}}}$

Hence, the extent of the Fresnel zone 508 at the wind turbine 510furthest from the radar is no greater than the length of the turbine'sblade. Accordingly, the extent of the Fresnel zone 508′ at closer windturbines 512 is even smaller.

It will be appreciated that wind farms may be very large and in thosecases it may not be possible for the radar transmitter and receiverarrays to be located within the proximity limit of all the windturbines. In this case the distance ‘D_(max)’ will be selected for thefurthest wind turbine, within the radar's field of view, for which theproximity inequality can be met. Accordingly, the distance ‘D_(max)’ maybe selected to maximise the proportion of the turbines within the fieldof view having a proximity limit ‘D_(P)’ at or beyond thetransmitter/receiver array.

It will be further appreciated that the holographic radar may comprise aplurality of transmitter/receiver arrays arranged and configured tooperate as part of an integrated system (as described for otherembodiments), and such that each wind turbine (or each of a substantialproportion of the wind turbines) at the wind farm are within the fieldof view of at least one transmitter/receiver array which is within itsrespective proximity limit.

The effective radar cross-section of the turbine blade is thussignificantly reduced when compared with radar located at a distance forwhich the Fresnel zone extends over the whole length of the tower orblade (i.e. when the incident wave approximates a plane wave with asource effectively at infinity) leading to a very narrow, high-gain,reflected beam (and hence large effective cross-section). For example,if the transmitter/receiver is at approximately 1 km from the turbine,and if the half-wavelength of the transmitted signal is approximately0.25 m, the radius of the Fresnel zone will be approximately 15 metres.Using these design parameters, therefore, the effective radarcross-section for a 30 m blade length (as seen by the receiver) will bereduced by a factor of approximately four relative to the long distancevalue of the cross-section.

As with the previously described embodiments, the receiver signalprocessor is configured to operate, at the distance within the proximitylimit, to correctly detect and identify wind turbines and other targetsby successfully discriminating between the radar returns from them. Theaccuracy of this process can therefore be enhanced because of thereduced effective radar cross-section of the wind turbines when comparedto other generally smaller radar cross-section targets, such asaircraft. Thus, targets of interest, and in particular targets which mayrepresent a threat, can be rendered easier to detect by appropriateconfiguration of the holographic radar to operate within the proximitylimit of the wind-farm.

It will be appreciated that a radar configured to operate within theproximity limit could be located at the wind farm itself (as describedpreviously) or at a distance from it (as exemplified in this embodiment)which is still within the proximity limit.

Advantageously, the holographic radar of this embodiment is configuredto operate within the proximity limit as close to the turbines of thewind farm as reasonably possible (to minimise the observed radarcross-section) whilst ensuring that the field of view of the (or each)radar transmitter/receiver array covers all the turbines for whichdetection and discrimination via the receiver array are required (and isnot obscured for example by a turbine tower).

Holographic Radar for Large and Small Turbine Arrays

In some wind turbine installations, a turbine array forming a wind farmmay extend a distance which is comparable with the height to which it isnecessary to detect and identify targets such as aircraft. In such casesit is particularly advantageous for the holographic radar to cover theentire area of the turbine array.

An embodiment of holographic radar suitable for covering the entire areaof a large turbine array is shown illustratively in FIG. 19 generally at600. The holographic radar is configured generally as described forprevious embodiments and, like previous embodiments can be configured asa stand-alone radar or as part of an in fill radar system. In thisembodiment, however, the holographic radar comprisestransmitter/receiver arrays 602 comprising four substantially planarantenna faces, pointing just above the horizon (for example between ˜5°and ˜45°, typically ˜20° or 30°) and in four orthogonal directions inazimuth (thereby covering substantially the entire field of view).

Whilst FIG. 19 shows the antenna arrays 602 physically pointing abovethe horizon it will be appreciated that a similar effect could beachieved by steering the transmitter/receiver beams appropriately from asubstantially horizontally pointing array. It will also be appreciatedthat whilst turbine towers provide an advantageous location for sitingthe transmitter/receiver arrays, the arrays could be sited in anysuitable manner including, for example, facing across the wind farm fromlocations at or beyond its extremities (for example within the proximitylimit described previously) as opposed to from within the wind farmfacing outwardly. Furthermore, the transmitter/receiver arrays could belocated on any suitable structure, for example an electrical sub-stationbuilding at or in the vicinity of the wind farm.

In other cases a wind farm may be very small comprising only a few windturbines or even a single turbine. However, even a single turbinerepresents a potential hazard to the successful and safe operation ofair traffic control or air defence radar.

Vertically Facing Radar

An embodiment of the holographic radar which is particularlyadvantageous for a small array/single turbine is shown in FIG. 20generally at 650. The holographic radar 650 is configured generally asdescribed for previous embodiments. In this embodiment, however, theholographic radar 650 comprises at least one transmitter/receiver array652 facing in substantially a vertical direction as opposed to justabove the horizon. The holographic radar in this case is configured forthe detection of a target 658 (such as an aircraft) flying in agenerally conical region 654 extending outwardly above a single turbine656 (or small wind turbine array).

It will be appreciated that although a substantially vertical directionis described for this embodiment, the upwardly facing receiver arraycould be arranged to face at any angle between about 45° and 90°, togive a field of view extending both vertically and towards the horizon.A receiver array that is arranged at such an angle may form part of awider radar system comprising a plurality of similar receiver arraysconfigured to provide an upwardly pointing field of view in differentdirections around the single turbine/wind farm. For example, a similararrangement to that shown in FIG. 19 may be employed in which each ofthe four receiver arrays point at an angle of at least 45°.

Thus, the holographic radar in this embodiment is different toconventional arrangements for air surveillance radars which lookoutwardly at the horizon to detect an incoming target such as aircraftearly so that it can be tracked and, if necessary, pre-emptive actiontaken (such as warning other aircraft in the vicinity, raising a threatlevel etc.) as soon as possible.

Whilst this embodiment can be configured as a stand-alone radar in aparticularly advantageous configuration it is configured as an in fillradar for a larger air traffic control, air defence or othersurveillance system as described previously.

The holographic radar of this embodiment may be integrated with aholographic radar according to the previous embodiment. In such anembodiment the radar may be configured to survey a combined regionextending upwardly from the vertically facing array (as described forthis embodiment) and outwardly from the arrays facing just above thehorizon (as described for the previous embodiment). This beneficialarrangement allows the roughly conical region above the wind farm, whichis not covered by the horizon pointing arrays, effectively to be ‘filledin’ by the vertically facing array. Such a system thus provides abeneficial arrangement for tracking an approaching target, such as anaircraft, both as it approaches and as it flies directly over a windfarm (large or small).

Time-Frequency Transformation and Beamforming

In FIG. 21 a functional block diagram of circuitry/signal processingmodules suitable for implementing the receiver signal processor referredto in any other embodiment is shown generally at 700. In this embodimentsignals are received by the receiver elements of a receiver array 702(which may be any of the receiver arrays generally as describedpreviously). The signals received by the receiver array 702 receivepreliminary RF processing by an RF processing circuit/module 704 priorbeing transformed in frequency to an intermediate frequency by an IFgeneration circuit/module 706. The analogue outputs of the IF circuitare converted to digital outputs by an analogue to digital (A-D)converter circuit/module 708.

It will be appreciated that although this embodiment is described withreference to receiver ‘elements’ the description is also generallyapplicable to the case of receiver sub-arrays each comprising aplurality of elements (as described previously).

A digital beamformer 710 (typically comprising an appropriate signalprocessing circuit or software module) forms multiple beamssubstantially concurrently (for example, one for each element), in thedesired directions, from the outputs of the A-D converter 708 usingappropriate phase and/or amplitude weightings. It will be appreciatedthat although a digital beamforming circuit/module is described (andshown in FIG. 21) the beams may be formed prior to analogue to digitalconversion at the RF or IF stage using appropriate analogue beamformingcircuitry, for example circuitry comprising phase modulators.

The beam outputs from the beam former 710 are then subject to variousforms of signal processing to support the detection and tracking oftargets, which generally includes a form of Fast Fourier Transform(FFT). The signal processing will now be described, by way of exampleonly, with reference to a specific holographic radar example in whichthe receiver comprises an array of 288 elements, the radar is configuredto detect targets in 256 range bins at a range gate rate of 2.56 MHz,and the FFT is of 1024 points. It will be appreciated, however, thatother configurations are possible, for example, in which the radarreceiver has a different number of receiving elements, is configured todetect targets in a different number of range bins and/or at a differentrange gate rate. Similarly the FFT may be of any suitable number ofpoints.

For a receiver array of 288 elements approximately 288 beams aregenerated by the beam former 710 which may be a randomly programmablebeam former. Accordingly, for a randomly programmable beam former, thebeam will generally complete four multiplication operations per element,per beam at the range gate rate. This equates to approximately 0.85Tera-operations per second (288 elements×288 beams×4 operations per beamper element×2.56 MHz range gate rate). Alternatively a Fourier Transformprocess may be used to form a regular series of beams (e.g.cosecant-evenly-spaced) more efficiently.

An FFT module 711 is configured to carry out complex FFTs on the beams.In this example the FFTs are carried out at approximately 10 Hz for a10.24 kHz pulse rate although it will be appreciated that the FFTs maybe carried out at other frequencies for other pulse rates. This equatesto approximately 20 Giga-operations per second (256 range bins×288beams×10 Hz FFT rate×4 operations per beam per range bin×1024 points inthe FFT×In(1024)).

A migration processing module 712 is configured to form a migrationsurface (e.g. a rang/range rate surface) for each beam and range andrange rate sub-beams are formed using the FFT elementary outputs.

A target detection module 714 is configured to detect any targets in oneor more of the migration surfaces and a positioning module 722 isconfigured to determine the position of each detected target usingamplitude monopulse measurements in each beam. Further processing isthen carried out as indicated at 724, for example, to store targetinformation, to identify wind farm related clutter, to classify targetsof interest etc.

It will be appreciated that positioning could be carried out moreaccurately using phase monopulse measurements as indicated in thealternative branch 716, 718, 720. However, this approach can be resourceintensive as it requires the beams to be reformed by module 716 (whichmay be the beam former 710 or part thereof) and hence the FFTs to berecalculated by module 718 (which may be the FFT module 711 or partthereof) before a positioning module 720 can calculate the position ofthe detected target using phase monopulse.

Time-Frequency Transformation Prior to Beamforming

In FIG. 22 a functional block diagram of alternative circuitry/signalprocessing modules suitable for implementing the receiver signalprocessor referred to in any other embodiment is shown generally at 750.As with the previous embodiment, in this embodiment signals are receivedby the receiver elements of a receiver array 752 (which may be any ofthe receiver arrays generally as described previously). The signalsreceived by the receiver array 752 receive preliminary RF processing byan RF processing circuit/module 754 prior being transformed in frequencyto an intermediate frequency by an IF generation circuit/module 756. Theanalogue outputs of the IF circuit are converted to digital outputs byan analogue to digital (A-D) converter circuit/module 758.

Unlike the previous module, however, complex FFTs are then carried outon the outputs of the A-D converter 758 by an FFT module 761, prior tobeam formation. For FFTs carried out at approximately 10 Hz (for a 10.24kHz pulse rate) this equates to approximately 20 Giga-operations persecond as calculated previously (256 range bins×288 elements×10 Hz FFTrate×4 operations per element per range bin×1024 points in theFFT×In(1024)). Thus the FFT's form a migration filter for each elementprior to beam forming.

A beam former 760 then forms multiple concurrent beams, in the frequencydomain, in the desired directions, from the FFT outputs usingappropriate phase and/or amplitude weightings. Typically, for example,the beam former 760 will first form ‘fan-in-elevation’ azimuth beamsbefore forming the elevation beams. In the case of a randomlyprogrammable beam former, the beam former will generally completeapproximately 0.87 Tera-operations per second (288 elements×288beams×256 range gates×4 operations per beam per element per rangegate×10 Hz FFT rate×1024 FFT points). Alternatively a Fourier Transformprocess may be used to form a regular series of beams more efficientlyas discussed previously.

A migration processing module 762 is configured to form beam migrationsurfaces (e.g. range/range rate surfaces) for each beam, and a targetdetection module 764 is configured to detect any targets in one or moreof the migration surfaces, as generally as described previously. In thisembodiment, however, a positioning module 762 is configured to determinethe position of each detected target using phase (as opposed toamplitude) monopulse measurements. Whilst this still requires the beamsto be re-formed it does not require the recalculation of the FFTs thatwould be the case if the previous embodiment were adapted to calculateposition using phase monopulse measurements. Accordingly, the beams arereformed (either different FFT beams, or beam pairs with knowncoefficients) by module 766 (which may be the beam former 760 or partthereof) based on stored outputs of the FFT module 761 without requiringresource intensive recalculation.

The use of phase monopulse can be advantageous over the use of amplitudemonopulse because it is easier to calibrate (for such receiver arrays),being an element-oriented single parameter, rather than a beam-oriented2-D plot. Phase monopulse measurements are also generally more accuratethan amplitude monopulse measurements.

It will be appreciated that although the embodiments described hereinare described primarily with reference to wind turbines, wind farms andthe like the radar systems, methods and associated apparatus has manyother applications including application in other cluttered and highlycluttered environments as described previously. In the context of windfarms it will be appreciated that the holographic radar (as describedherein) may be used for analysing wake effects of turbines and inparticular wind-wake effects especially for use in maximising the energygeneration potential of a wind farm.

Each feature disclosed in this specification (which term includes theclaims) and/or shown in the drawings may be incorporated in theinvention independently (or in combination with) any other disclosedand/or illustrated features. In particular but without limitation thefeatures of any of the claims dependent from a particular independentclaim may be introduced into that independent claim in any combinationor individually.

Statements in this specification of the “objects of the invention”relate to preferred embodiments of the invention, but not necessarily toall embodiments of the invention falling within the claims. Referencenumerals appearing in the claims are illustrative only and the claimsshall be interpreted as if they are not present.

The description of the invention with reference to the drawings is byway of example only.

1. A radar system for discriminating between sources of radarinterference and targets of interest, the system comprising: means fortransmitting radar signals into a region; means for receiving returnsignals of said radar signals returned from within said region; andmeans for processing the return signals to discriminate between returnsignals returned from a first object and return signals returned from asecond object wherein said return signals from said second objectcomprise Doppler components and interfere with said return signals fromsaid first object; wherein said radar system is operable fordiscriminating between said return signals when said return signals arereceived at a distance from said second object which is less than aproximity limit based on the geometry of the object. 2.-103. (canceled)